Related Local Links:
Beating the 50 lpmm Limit
Film Resolution (lpmm) Pages
How Much Quality Do You Need?
Resolution Issues
Scanners and Resolution

Related Links:
6 MP 24mm x 36mm chip
Buy Dem Digitals (a humorous plea from a used lens buyer..)
CMOS vs. CCD sensors [12/2002]
Defocused Lenses Improve DOF by Tenfold (6/2003)
Digimarc Image Degradation (Ken Rockwell) [9/2002]
Digital Camera Preview Pages
Digital Back vs. Film
Digital - Devil's Handmaiden
Digital's Dirty Little Secret
Digital One Use Storage (like film)
Digital Photography - Is it Worth It? (Peter Williams) Film Scanners Pages [7/2001]
Film vs. Digital (Dante Stella) [11/2003]
Film vs. Digital (Ken Rockwell) [9/2002]
Flooded Digital Camera Produces Unique Photos (Farrell Photogr.) [6/2003]
Foveon X3 Sensor Images
Fungus Eats CDROM [7/2001]
Fungus eats CDROM Disks (really)
Kodak on antialiasing filters
Lenslets create Credit Card Thickness Lenses (6/2003)
Lifespan of Digital Files by Julian Jackson
Low Cost Medium Format Digital (scanning) [12/2002]
Megapixel Myth (Ken Rockwell) [9/2002]
R.P.Digital FAQ...
Resolution of 3/6/12 MP vs. Film (film > 12 MP)
Scan Tips Pages [7/2001]
Scanning Faq [7/2001]
Should You Buy A Digital Camera? (Ken Rockwell) [9/2002]
Slave Flash Trigger for Digicams
Sony Memory Stick Incompatible with Older Designs [12/2002]
Vertical 3D Chips

Why it Matters

The second consequence is economic. Many people are spending a great deal of money on digital cameras, computers, software, and printers in the hope of "saving" money over film based cameras. Unless you are doing a huge volume of photographs, you probably won't recover the depreciation losses from going digital over the lower cost of film at typical amateur shooting volumes.

The final reason is skills related. Being a good digital photographer involves an entire skill set of mainly computer and software related skills that aren't part of traditional photography. These skill sets have a difficult learning curve, and assume much mastery of underlying computer skills also unrelated to traditional photography.

Yet we all only have so much time for doing photography. Will it be spent in front of a computer, constantly learning new software and hardware issues, or will it be spent behind a camera and lens? Will your photography get better from studying computer manuals, or from studying the images of great photographers and other artistic sources?

Where Digital Comes Out Ahead

Question: What is the major photographic benefit from using digital cameras?

1. near zero cost to make photos
2. convenience of making photos
3. greater depth of field in photos

If you said enhanced depth of field over larger format film based cameras, you are right. The small size of film sensors corresponds to smaller film formats, and gives similar DOF benefits. The chip diagonals for common sensors run from 6-8mm (webcams) up to 14-20mm or so. Recall that a 50mm lens for 35mm film has (2 stops) more effective DOF than a 75mm lens on a medium format TLR. By similar ratios, digital cameras can have even larger DOF than most 35mm users are used to seeing. On the other hand, if you want to isolate a subject from its surroundings, that huge DOF can be a serious problem!

The biggest selling point of digital cameras is convenience. You can get a digital image quickly and conveniently, without the need to develop film and scan an image. You can print out a color image from many color printers quickly, without the need to maintain or have a color darkroom. You can preview your shots in the field, and delete any that you don't want and retake those you do want. But this instant feedback isn't new. Many of us have used polaroid film backs on medium format cameras for decades.

Where Digital cameras often fall flat...
• fine contrast detail (over 40 lpmm, due to anti-aliasing filter in digicams)
• shallow depth of field shots (portraiture..) due to great DOF (small sensor..)
• long duration exposures (moonlight...) often problematic
• very fast shutter speeds (freeze-motion) often lacking
• color smearing (three/four sensors per color pixel spreads or smears colors..)
• cold weather - batteries fail, motors and lubricants gum up etc.
• wet weather - possible electronics shorts
• rough handling - river rafting, wars.. - vs. Nikonos or Nikon F "bricks"
• dusty or sandy areas - dust in drive filters, internal guts (via card port..)
• remote area supplies - batteries? solar panel rechargers? storage media (Nepal?)
• repairs - only at mfger, forget local repairers (vs. watch and camera repairers)
• Lighting control - off-camera lighting interfaces costly/clumsy if available
• small screen sizes - hard to really see screen esp. if bright light on LCDs
• lack of focusing aids (split prism, grid for architecture, variable diopters..)
• shutter lag - considerable with AF designs and storage sequences
• sequence photography - length of bursts limited (vs. roll of 36 to 250 exposures)
• sequence photography - speed of shots limited by storage speed and image size etc.
• range of equiv. speeds is limited - vs. film 0.8 ISO to 6400 ISO film options
• range of light levels may be less than dynamic range of film
• smaller less massive cameras more subject to camera vibration and shake
• no DOF depth of field indicators or preview option
• operation with telescopes and microscopes is problematic
• tripod unfriendly - hard to do rule of thirds with central AF sensors on tripods
• operation with bellows and extension tubes may be problematic (no couplings?)
• smaller lenses can be slower, making AF and other features problematic in dim light
• problems with ultrawide lenses and fisheyes esp. with 1.6X and similar small chips
• software bugs and glitches, lack of workarounds
• all or none operation - glitches can be fatal vs. problem in 35mm SLR, can still shoot
• lack or difficult of manual settings and over-rides (ambient light meter etc.)
• red-eye from on-camera flash which is too weak to be very useful anyway
• complexity of manuals and operation - manuals may weigh more than cameras
• symbols (icons) often cryptic, complicated by multi-modes and interacting controls

Some of these problems are generic to all current digicams. Others are "fixed" in some degree in the high end cameras, esp. with larger sensors (24x36mm) and SLR camera mounts.

How Many Megapixels to Equal Film?

What size DSLR sensor do you need to equal the resolution of a mid-speed ISO 100 film? Kodak suggests you need at least 24 Megapixels. Popular Photography and Imaging Magazine real world tests suggest you need a 40 megapixel sensor. An AFIPS scientist's tests suggest that 54 megapixels is possible with mid-speed 35mm film. Let us be conservative, and go with the "real-world" tests estimate of 40 Megapixels per Popular Photography magazine tests with mid-speed film.

What would you expect for Velvia, a noted fine grain slide film with ISO speed of 50? Based on the AFIPS researcher's table above, I would expect about 2 1/2 times the ISO 100 megapixel estimate. We are using a conservative 40 Megapixel estimate for ISO 100 film (from Popular Photography real world tests). So I would estimate 2.5 times 40 Megapixels or 100 Megapixels as the expected value for Velvia in 35mm. For even finer and slower ISO 25 films, that value could be as high as 125 Megapixels on 35mm, as suggested by the AFIPS researcher's table quoted above.

So why do talented and experienced film scanners like Roger Clark (see posting) get only 200 Megapixels for 4x5" scans of Velvia? That's about 10 Megapixels per square inch, or about 14.4 Megapixels for 35mm film (24mm x 36mm = 1.4 sq. in.).

My short answer is prosumer scanners are unable to capture the full detail stored in film. If you switch from a CCD based scanner to a photomultiplier tube drum scanner, you get hugely more high frequency and high quality image data from film scans. Medium format slide scans often fill an entire 600+ Megabyte CDROM with a single scan, enabling 3x5' enlargements (that's feet, not inches ;-).

A related question is why do digital scans produce such grainy images from film? Again, the short answer is that the charge coupled device sensors are reduced in size to provide high density scans. But smaller sensors are troubled more by noise. The CCD scan generated noise interacts with whatever grain is on the film to cause a large increase in the apparent grain. This problem is different for different films, depending on their grain structure, and to some degree on the specific scanner in use. But if you scan the same film with a drum scanner, you get much less noise and a nearly grain free image from fine grained films.

Why is this an important observation? Your 6 Megapixel DSLR will only provide 6 MP images. The same image shot on film is really being recorded on a 40 megapixel sensor (ISO 100 film) or even a 100 Megapixel sensor (i.e., Velvia ISO 50 film). As scanners improve, or with a $10-15 drum scan today, you can get this image quality from film. Most of us have seen and admired the grainless enlargements from slide film called "laser prints" on Cibachrome or Ilfochrome papers, often to poster and even wall sized prints. As future film scanners improve, the results achieved will be equally impressive for film users in the future. But you will be able to have your film scanned on higher quality scanners, and get larger enlargements and higher quality images from film than any DSLR can now provide.

What about all those claims of digital users who have "tested" their DSLRs against film and determined that film produces less quality than their 3.1 Megapixel camera, or new 6 megapixel camera or whatever? Who do you think knows more about testing film and digital cameras: these amateur testers or Kodak and the lab technicians at Popular Photography's testing lab?

Since medium format film is circa 3.8 to 4.2 times the area of 35mm film format, it follows that medium format is 3.8 to 4.2 times the megapixel rating of the 35mm films, depending on the specific film ISO speed and particular format (e.g., 6x6cm vs. 6x7cm). Likewise, large format such as 4x5" is roughly 14 times the size of 35mm film. So it will take at least a 512 Megapixel sensor to equal 4x5" film at ISO 100 speeds!

Digital Chip Surprises

Foveon's 16MP chip uses 0.18 micron technology, versus 0.35 to 0.5 micron features for their current 3.1 and 6 MP chip competitors. But we are near the end of what current CMOS technology can do, now and in the foreseeable future. The flip side is that at 16MP, the results will be good enough for most current disposable camera and point and shoot users. So there may also not be much market pressure for higher density chips, just like there aren't many large format users today either.

Foveon's 16MP chip is also 22 by 22 millimeters square(!), which is a problem for rectangular format fans such as 35mm SLR's familiar 2:3 ratios. Simple geometry suggests the 22x22mm chip will only image 56% of the 24x36mm format provided by conventional 35mm SLRs. Using your 35mm SLR lenses would result in a chip-based cropping of the 2x3 rectangle to a 1:1 square - surprise! Your wide angle lenses will become much less wide too after this digital cropping. Things will be worse on medium format, with the 22x22mm chip only imaging 16% of the format area. So a very wide and heavy 40mm Hasselblad lens will act like a 100mm short telephoto lens. Too bad for us wide angle fan(atics)!

Foveon's 22x22mm square chip has 16.8 million sensors, representing 4,096 x 4,096 sensors on a square grid. Given 4,096 sensors on each 22mm axis, you must have circa 186+ sensors per linear millimeter (i.e., 4096/22=186+). You need two rows of sensors to image a line (black/white). So 186 sensors per millimeter corresponds to 93 lpmm (186/2). The sensor can only achieve such maximum resolution if you were to optimally and carefully align the lines with the image grid. In practice, the alignment would be more or less random, and you would only get about half the maximum resolution or about 45 lpmm (cf. Nyquist sampling limits).

Strange as it may seem, the big problem with most optical systems for digital sensors is they are too good. Lenses have aerial resolutions that can run as high as 300 to 600+ lpmm. A low pass filter setup is used to reduce this high frequency response so it doesn't cause problems (such as aliasing). You may also have an infrared filter in front of your sensor, so it isn't adversely affected by invisible IR radiation to which many silicon based sensors are very sensitive.

Another interesting problem with using existing 35mm SLR lenses designed for film use is that digital sensors are not flat. Instead, the digital sensor's active site is inside a sensor "well" or cavity. The walls of this cavity keep light from the side from easily reaching the sensor below. The ideal lens for digital sensors would provide a parallel bundle of rays (rather than converging) from the rear of the lens onto the sensors. Light from very wide angle lenses in particular come in at a severe angle to the chip surface from many points on the lens. This light can be blocked, providing yet another problem when trying to use current 35mm SLR lenses on digital sensors.

This problem is currently masked by the small chip size of most sensors (e.g., 22 by 22 mm for the Foveon 16MP chip). The chip is only seeing the center of the image circle, often after passing through additional optics plus the front filters (IR, low pass..) on the digital chip surface. We should mention that filters behind the lens cause focus shifts (equal to 1/3rd their thickness, usually) (see Filter FAQ). Other problems like flare can be made worse by rear mounted filters too.

Medium format users would be cropping to the same 22mm x 22mm chip size. So even if we could get such a 16MP chip in a 6x6cm digital back, we would get only 1/6th of the image, and at less than optimal resolution. You can add optics to focus the 56x56mm image onto a 22x22mm square. But besides the cost, you also have to expect rather lower total resolution. The resolution of your optics onto the 22x22mm square is already marginally low, and doesn't get full benefit from the chip's potential resolution. Since the big advantage of medium format is larger film area, using a small chip defeats this expectation. You might as well use 35mm SLR optics and save the weight and cost, since the chip size and optics are the limiting factors.

My bet is that the 16MP and larger chips will be made as small as technology limits (chiefly noise) will allow, and the lenses will be sized to match. Fortunately for users, small lenses such as those used for microfilm cameras can have very high resolutions at relatively low cost (e.g., 250 to 350+ lpmm). It is much easier to improve a small lens and minimize aberrations than in a large one. On the other hand, diffraction becomes a big problem with small lenses very quickly (e.g., past f/2.8).

So I would predict very small, lightweight, and fast optics. I think fixed wider angle lenses will be popular, while "zooming" will be done digitally using interpolation. The lenses will be fast because the smaller size of the sensors will make it hard to avoid noise unless you have a lot of signal (light). Sheets of microlenses looking like bug eyes will help focus light from the entire chip surface onto the limited light sensitive area of the chips (e.g., 30% of chip area).

I am not saying 24x36mm or even 56x56mm (6x6cm) or larger chips won't ever be made. I am betting that those larger chips will be custom production runs, for a very limited (in chip maker terms) market of 35mm and larger camera users. The really low cost mass produced chips will not be aimed at the relative handful of us owning 35mm SLRs. The current Foveon 16MP CMOS chip maker (National Semiconductor) CEO is even talking about millions of cheap 16MP sensor chips added to portable videophones and other gizmos including disposable 16 MP cameras (actually, recycleable is a better description).

Now do you think that you are going to lug around that medium format camera and lenses, or your ten pound bag of 35mm SLR bodies and heavy zoom lenses and tripods, or a six ounce $100 16 Megapixel recycle-able camera? Remember, both will deliver the same 16MP resolution. The tiny lens on the disposable camera may even have lower distortion than those oldie Zeiss or Nikon optics which weigh much more. After all, you can use digital technology to map the distortions on the lens and then correct for them in software (but not in film). Do you really think you will carry around all that obsolete glass, or just use the 16MP sensor in your video digicam or videophone and upload directly to your server and home printer?

My bet is that another ten years will have most current 35mm and larger film format cameras seem as heavy and unappealing as a wooden Kodak 5x7" view camera.

But the bright lining in this digital cloud is that film is likely to remain the high quality choice in the future as it is today. The 16MP chip developer, Carver Mead, is quoted as saying that it is unlikely that digital chips will gain much additional resolution and may already be pushing the limits. So to get more resolution, they will have to make bigger chips, but that will cost much more due to lower chip yields and limited market demands. Digital camera chips are really only affordable when they are mass produced, and that requires a mass market. Few users today have a need for quality beyond 35mm, as lagging medium format and large format market shares show. So folks who want a high quality image and larger prints will need to turn to film to supply that quality for at least the foreseeable future.

Chip Benefits

Most films are limited to a dynamic range of 7 or 8 stops in practice, for a light range of 1:128 or 1:256. Silicon sensors are capable of much greater dynamic ranges. However, most prosumer digital cameras limit the range of response to a rather narrow range for best picture quality. Under challenging conditions of lighting or subject matter, you may have to reshoot after deleting the bad shot seen on the tiny camera back mounted LCD screen. Many cameras have only a limited ASA or film speed rating range in which this response range can be shifted.

By contrast, you can select film speeds from ASA 0.6 to 32,000. Film reciprocity makes it possible to adjust exposures (and filtering) for longer exposures, up to some hours long for moonlit landscape photos. You can't do this with digital sensors, unless you intend to cool them in liquid nitrogen. The sensors build up noise quickly, so good images can only be achieved for a limited range of short time durations. So certain kinds of long exposure time photography can not be done with digital cameras due to these sensor noise accumulation problems.

Silicon sensors are very much more efficient with low level light than film (however, a 10X or 1,000% faster fine grain film is in the offing, see below). Here again, most digital cameras limit the range of film speed equivalents to only one or two film speeds. Most modest cost consumer digital cameras have a fixed film speed. So what could be a benefit of digital cameras is lost against the ability of film users to pick a range of film sensitivities up to 6,400 ASA and beyond. Making this worse are the new print films with multiple light sensitivity ranges from 100 ASA to 1,000 ASA/ISO ratings in the same film.

Because of their dynamic range, silicon sensors should have a wider latitude of exposure in theory than film. Unfortunately, in practice most digital consumer cameras have an optimal lighting range for getting good pictures that is not very large. By contrast, color print and black and white films have a rather wide range of exposure latitude over which it is possible to get an acceptable print out of the film. Even slide films have a one or more stop range of over or under exposure.

Most consumer digital cameras respond poorly to under or overexposure. Most models seem to work best with a limited range of light (e.g., sunny f/16 daylight conditions). The flash units on lower end digicams are often automatically triggered to supplement even relatively bright light levels. The higher end and more costly digital cameras provide more of the potential range of silicon sensor benefits to the user. But even here, the film user can choose from a wider range of ASA film speeds and often greater latitude in under or over-exposures too.

Digital camera users often proclaim that they can make as many exact digital copies as they wish. That's important, since the rapid obsolescence of storage and computer and digital camera technology means a new generation of each is out every 12 to 18 months. Many digital users ignore the costs of these upgrades. They also ignore the time and labor it takes to organize, backup and convert their images to the latest formats and storage systems. Backups are particularly critical since destructive viruses can potentially destroy your entire online digital photo collection.

If you maintained your digital photos online, this practice would help backup your images automatically via the file server backups. But most web sites are limited to 10 megabytes storage, or 100 megabytes at best, with charges often related to the number of stored megabytes. For high resolution image scans (e.g., TIFF), it won't take many images to use up 10 or 100 megabytes of storage. So your costs for maintaining these files online represent yet another hidden cost of going digital for many users.

You can use gigabytes of local storage instead. But you have to have a way to backup those gigabytes and do so often enough not to lose data in system or virus related crashes. Digital files are subject to the various risks of film and prints (fires, floods..) but also add their own risks. Having seen lots of virus related crashes this last year alone, I suspect that digital photo files are much more at risk than traditional film and print media, especially in the home (non-professional backups) environment.

Aliasing - Or Why Film Grain is Superior to Grainless Digital Cameras

Digital sensors are subject to a rather troubling problem called "aliasing". Aliasing can occur because digital sensors are precisely deposited arrays of sensors and silicon features in a regular grid structure. This grid pattern can interact with regularities or patterns in the image to produce a series of artifacts on the digital imaging. The most familiar example are the Moire patterns and color fringing artifacts.

To try and prevent this troubling problem, most higher end digital cameras have extensive image processing and aliasing detection software. This software tries to guess when you have an aliasing problem (versus when you are using a diffraction grid filter, say). Then the software tries to guess how best to process the image to remove the aliasing artifacts in the image. Depending on the software and aliasing image, the result can be very good to very poor.

The solution to an aliasing problem is simple. You need to convert the regular array of identically sized sensors into an irregular, randomized array of sensors of different size. This solution to digital aliasing is precisely what we have with film grain - an irregular array of different sized grains of light sensitive elements.

Film is Toes and Shoulders Above Digital

Many digital camera users are familiar with the problem of "blown" highlights. Digital images often have to be very carefully exposed to prevent or control highlight areas. The problem is particularly acute with digital video. The toe and shoulders of film's characteristic curve enables errors in film exposure to be compensated after exposure by shifting up or down the characteristic curve into these non-linear response regions. The flattened change in slope of these non-linear regions means large errors produce only slight increases in shadow or highlight response.

So while film and digital sensors may have similar dynamic range in many cases, digital values are inherently linear, and so block up when maximum levels are reached. With film, the shoulder response is non-linear. A stop or two of over-(or under) exposure can be compensated for in processing or copying. With digital, blown highlights or blocked up shadows can not be restored.

Sensor Area - Another Advantage of Film

The size of film grain is a randomized bell shaped curve distribution around some average grain size which is different for each speed and type of emulsion. A typical film grain for mid-speed film might be circa one micron in area.

The first thing you might conclude here is that you would get, on average, nine film grains with a total area of 9 microns (one micron area each) into the same size space as one silicon sensor, namely 9 square microns based on a 3 micron square sensor feature size.

So even if you could build a silicon sensor array the same size as film, it would have much lower resolution than film because of the large size of most silicon sensors versus film grains.

Stacking the Deck - Layered Emulsions versus Silicon Sensors

Film cheats by layering the different color filters and emulsion layers one on top of the other in the typical color film emulsion. This trick is impossible in making a silicon sensor, since they are a planar grid in two dimensions. You can't stack silicon sensors vertically, because the top layer would block light from reaching the sensors underneath it.

Moreover, film is an analog medium. A film grain may overlap another grain by any percentage value, not just a binary "1" or "0". So the actual range of densities in a 20 micron thick typical film emulsion is highly variable over a wide range. A particular digital sensor of say 12 bits accuracy can register 2^12 or 4,096 tonal values from a 36 micron square sensor site. But film's analog variability is much higher, given 20 or more grains which can overlap by any of a continuous range of values from 0% to 100%.

The layered nature of color film emulsion lets us achieve a remarkably small area for our color sensors. If the average grain size is our typical one micron area, it will be the same in each color layer (red, green, blue..). These color grains can be superimposed on top of each other to create the individual color elements in the final color Kodachrome slide.

Why did I say Kodachrome slide above? Because Kodachrome slide films replace the individual film grains exposed in each layer with a colored dye of essentially the same size. Other E-6 color slide films such as Ektachromes and Fujichromes are simpler to process, but the resulting color dye blobs are somewhat larger than the original exposed film grains. Now you know why Kodachrome slides are so sharp. The Kodachrome chemistry is different, and the smaller color blobs result in higher resolution and sharper slides for Kodachrome slides. Newer color print films have finally equalled and in some cases exceeded non-Kodachrome slides in potential resolution recently.

As a result of this film processing effect, we have to de-rate the advantage of the small one-micron sized film grain area somewhat. So we won't claim that our one micron film grains are 36 smaller than the average digital color sensor area (a composite of 4 color filter masked sensors of 9 microns area each). Instead, we'll suggest that color film enjoys a 20-fold or better advantage in smaller color sensor area against digital camera sensors.

Digital versus Film Color Bias Issues

Is white balancing a real plus for digital cameras, as often claimed? Probably not, at least for many film using photographers who make a point of waiting for the "magic hours". These "magic hours" right around sunrise and sunset are called "magic" precisely because of the warm and grazing lighting effects in landscape and similar travel photography. I regularly use yellowish #812 or #81A/B filters to provide an extra bit of warmth in the bluish shade of daylight photographs. At other times, the blue cast of tungsten film mis-used in daylight can be used to emphasize the coldness of winter scenes, as one popular cliche example. Photographers can also "tune" their lens coloration with color correcting gel filters as desired.

The above quote was highlighted by a series of tests by the BJP article authors of sundry digital cameras. Not only did the color response vary between different sensors and brands of hardware and software in digital cameras, but it also varied between different ISO settings on the same camera! I found the differences in color casts between different film and digital cameras to be surprisingly large. I was especially surprised by the large degree of color bias in the sample portraits of some digital cameras, especially against other more neutral models (e.g., Nikon D series versus Olympus E series).

Many film users make a point of selecting particular films, with known color biases, to match particular subjects. For example, I used to shoot Kodachrome 25 for its fine grain and response to reds found on coral reefs in my underwater photography classes. Fujichrome did far better with greens in tropical scenery, so I carried some fuji films for their color signature too. Konica film was consistently picked as having the best skin tones in viewer tests. The new Kodak 100EVS and similar films have color biases tuned to subjects such as portraiture (skin tones..) or landscapes (e.g., pumping up color saturation for fall foliage color shots).

What does this matter? Simply stated, different cameras "white balance" to different degrees and biases, resulting in particular color casts in their images compared to other brands and film. Some digital users claim that they can adjust the color balance of their digital images to mimic nearly any film color response. Maybe so, but it takes a lot of time in Photoshop to get this color balance convincingly right. Many digital cameras do offer a useful feature of bypassing the software color biases of different cameras by downloading the raw sensor data directly. But in these cases, the digital photographer has to provide the efforts needed to achieve the desired color biases and balances by over-riding the built-in camera software.

To my view, the color biases of film emulsions remain a useful creative tool. Evidently, digital camera users have felt so too. Digital camera users are now demanding similar standardized color biases settings as a menu option, similar to various popular film color biases such as Velvia. Similarly, digital image processing programs are vying to come up with such film like color bias palettes, again to provide this simple and effective way to match color bias to subject. If such color biases were not useful, then why are digital camera users demanding them as new features in cameras and image processing software?

Resolution Limits

In fact, you can calculate the maximum resolution of today's digital camera sensors based on the sensor size. When you do, you get a value of circa 55 lpmm as the maximum resolution of most sensors. Simply realize that a lpmm has to have a line of black dots and a line of white dots to tell it is a distinct line. Color sensors (with support and control lines) are typically spaced 9 microns apart, so two lines of sensors take 18 microns to make a line of black and white dots. From the math, 1/18 microns is 0.0555 lines per micron, times 1,000 microns per millimeter, yielding 55.5 lpmm. So the best case resolution of current day consumer digital camera sensors is typically around 55 lpmm.

In the real world, you need to consider sampling issues, as image data will practically never align with the sensor array in the theoretically maximum resolution best case. Generally, you only get about half the maximum value, or under 30 lpmm in our example. This resolution limit would be easily achieved with 800 ASA print film in the $8.95 disposable Kodak Max HQ cameras with two element plastic lenses (rated at over 30 lpmm in Popular Photography tests).

Image Quality - 6 vs 8 lpmm in the Print?

Many photographers are happy with less than 4 lpmm on prints which are viewed at a longer distance (e.g., 20 inches or more). Larger prints are often viewed from afar. So you can't detect the lower print resolution without getting up close (e.g., 16x20" at 20"...). If you masked off an 8x10" section of these larger prints, and looked at it from ten inches, you would see that the print quality is less than optimal (with experience). The effects of low quality minilab prints has led to a further erosion in the level of acceptable print quality too.

Most digital printers use a relaxed print quality standard as a way of expanding the size and area of an "acceptable" quality digital print. The math is again simple arithmetic. For 300 dpi printers, you divide 300 dots per inch by 25.4 millimeters per inch. The result is 11.8 dots per millimeter. Unfortunately, it takes a row of black and a row of white to make a line. So we have to divide that 11.8 dots per millimeter by 2 to produce circa 6 lpmm on the final print. So a 300 dpi printer is capable of producing nearly 6 lpmm quality prints.

To reach the Leica standard of 8 lpmm on the final print, you would need over 400 dpi (25.4mm/inch x 8 lpmm x 2 dots/line).

What does a typical 3 megapixel camera deliver? First, start by ignoring that many 3 megapixel cameras have really just 2.7 million usable pixels. We will also ignore that most digital sensors are not 8:10 aspect ratios. An 8x10" print printed full frame has 8" x 10" or 80 square inches of area. Dividing 3 million pixels from the digital camera by 80 sq. in. yields 37,500 pixels per square inch. The square root of 37,500 is 193 pixels per linear inch. In other words, a 3 megapixel camera can only produce 193 pixels of true color data when making an 8x10" print. But many color printers print at 300 dpi, or 600 dpi, 1200 dpi, or even 2400 dpi. So where are all those millions of extra colored dots coming from?

In practice, software is used to interpolate or project a smoothed set of data for the printer even when printing at a modest 300 dpi. As we calculated above for a 3 MP digicam, we have circa 37,500 pixels/sq. inch. For 300 pixels/inch, we need 300x300 or 90,000 pixels/sq. inch. We have only 37,500 pixels/sq. inch. In other words, the software is interpolating roughly 2 out of every 3 pixels in a typical 8x10" print at 300 dpi.

Now you know why digital prints have such a smooth and "creamy" texture to them. The vast majority of printed color dots are interpolated between the relative handful of actual or real color data points from the digital camera. The higher the printer dpi, the more dots and the more smoothing that goes on.

Given that 400 dpi corresponds to 8 lpmm, 193 dpi corresponds to less than 4 lpmm. So an optimally sized true 3 megapixel camera is delivering at best less than half the 400 dpi needed for a Leica quality standard print. Stated another way, a Leica quality print (at 8 lpmm) will have four times the resolvable image data on the same size print. That is quite a quality difference!

To get a Leica quality standard (8 lpmm) 8x10" print out of an optimized aspect ratio (4:5) sensor digital camera, you need not a 3 megapixel camera but more like a 12 megapixel camera (4 times more sensors). Assuming future 2:3 aspect ratio (corresponding to 35mm film's 24x36mm) digital cameras, a 16 megapixel camera will just about produce an 8 lpmm quality standard 8x10" full print on a 300 dpi color printer.

Squandered Silicon

Imagine a window screen in which each tiny square blocked 2/3rds of the light. You will still have an image, but it might be somewhat different from one in which 100% of the light is used to generate data. The actual point where a dark area and light area change over in the image may be incorrectly guessed by the software. Moreover, smaller light sensitive areas mean fewer photons are captured, and eventually the noise levels kill the quality of your digital image.

Some chips (such as RCA) use a matrix of microlenses over these smaller chip sensors. This trick helps the smaller sensor act more like their more efficient (70% or so) cousins, especially improving noise performance. But the flip side is that the smaller sensor size can't be used to improve the potential resolution of these chips.

Chips and Resolution Limits

One reason current sensors remain relatively large is that the overall system costs are kept lower by using cheaper lenses. This factor more than offsets the slightly larger silicon real estate used in making the larger chips. With a 9 micron sensor, we had circa 55 lpmm as a resolution limit. The sensor is smaller than 35mm film, so it is easier to make a higher resolution lens for it more cheaply than for 35mm film too. Most third party zoom lenses can easily deliver this level of optical resolution for the small size sensors of most current chips. So we are at a "sweet spot" where the chip size and required low lens resolution makes it cheap and easy to build current digicams.

Let us say you develop a 16 MP chip with much higher density in the desired 24mm x 36mm size format. Since today's 3 to 5 MP chips are slightly smaller than 35mm film (by 40% or so on), some of the higher density will come from larger chip area to reach 24x36mm sizes. The rest will have to come from higher sensor density. But there is a problem with denser and smaller sensors due to problems with noise in the small sized sensors and the limited amount of light hitting the smaller sensors.

Assuming you have roughly halved the sensor size, you would expect roughly twice the resolution you had before. Our old 9 micron sensors delivered at most roughly 55 lpmm. Twice that 55 lpmm value is 110 lpmm. For example, if the new X3 technology announced in Jan/Feb 2002 (see postings) enables stacking three red, green, and blue sensors on a single sensor site, then such resolutions might be achieved (at least in bright light)? For photographers using autofocus, our AF lenses can rarely exceed 50 lpmm with any consistency. Many low end consumer zooms may be challenged to deliver 110 lpmm resolution to the chip surface (especially in the corners).

I do need to point out that when we deal with lens resolution on film, it is the result of the camera, lens, and film resolution components taken together as a system. With most color films, the low film resolution limits (typically 50 lpmm to 80 lpmm) means that the film is more limiting than our lenses. Many lenses can deliver well over 200 and even 400 lpmm, with some high end optics (e.g., Leica summicron 50mm) hitting 650 lpmm aerial lens resolution. The problem is that these lenses are too good!

So digital camera designers have to put in a low pass filter, which cuts off the high frequency components. Those high frequency components are the crispy details and sharp elements in your image, and the image projected by a quality lens. None of this data gets to the digital image sensor, where it could cause aliasing and other problems. The low pass filter is basically a softening filter that is permanently mounted over your digital camera sensor to "dumb down" your lens to a low enough resolution compatible with your digital sensor. So you might as well be using a cheapy lens with most such low pass filtered digital sensors. The costly high resolution components of your pricey OEM lenses will be filtered out and lost anyway.

The flip side of this argument is that 35mm sized chip sensors can only deliver resolutions slightly better than today's 3MP to 5MP cameras using current 35mm lenses. At some point, you don't have enough light to provide a large enough signal to the sensors to overcome the inherent noise in the smaller sized sensors. Do you see the problem here?

How about a medium format sized chip? With a larger area, you could use current medium format lenses with a larger area chip to yield higher quality digital images (e.g., 16MP). The problem here is few folks have medium format rigs, and they are big and heavy. The larger chips would have higher rejection rates. The bigger chips would have more chances to have a defect on them due to their larger area. That spells higher costs too. To me, these observations suggest that the cost of custom digital backs for medium format will remain high for some time to come.

New Lenses for 16 MP Cameras

The other side of this issue is whether it will be worthwhile to have a digital back or digital film insert for existing 35mm SLRs and medium format rigs. None of the current 35mm film based SLRs nor medium format rigs are optimal platforms for a digital system. Only a few medium format cameras even have data links to their backs and lenses (e.g., Rollei).

Now suppose a 16 MP digital camera with a super high resolution microfilm format zoom lens weighs less than a pound with batteries and gigabytes of removable data elements. Thanks to mass production, it costs under $1,000. Do you really think you would lug around, let alone buy, medium format or 35mm SLR bodies to get much lower resolution images from the lower resolution 35mm or 6x6cm lenses? No, huh? Conversely, given you can have a 200 lpmm zoom microfilm format lens for $100 cost on your 16 MP digicam, will you really be bummed out by not having to carry around all those 35mm or 6x6cm heavy lenses? Hmmm? The only thing you are getting by using your 35mm or medium format SLR as a base for a digital camera is a poorly designed and heavy case. Your expensive high resolution lenses will be wasted as their high contrast and high resolution images are put through a low pass filter (acting as a softening filter essentially) to reduce your high dollar lens resolution to a low enough level to match the chip's limitations.

In short, I think we will see an interim design using the existing base of 35mm SLR lenses at or near their resolution limits on a digicam body for the 5 to 10 MP resolution chips. Medium format backs will continue to be specialty items at high cost, due to the small size of the market and its fractured nature (hasselblad vs. rollei vs. mamiya..). The future 16 MP digicams will probably use smaller high density chips mandating smaller high resolution optics which will obsolete 35mm SLR lens based cameras.

Financial Sanity

Digital camera users would have you believe that since you don't need film or minilabs to do digital prints, the cost of digital photos is effectively zero. Maybe so, but they must be stealing those 2CR23 batteries from somewhere. If you are used to replacing a mercury or silver cell in your light meter every 3 to 5 years, the battery costs for a digital camera can be a suprise. Even worse, if you need to use flash for many of your photos, you will be shocked at how fast even a small flash eats up lithium batteries. On one of my web cameras, I can take 250 shots per set of batteries (3 cents per photo), or 60 shots with flash. A typical mix yields circa 8 cents per photo for digital camera battery costs alone.

Naturally, you could use an external battery pack with rechargeable batteries and a charger, and carry spare batteries. Some digital pro cameras use AA rechargeable batteries, although many prefer the higher energy and much higher cost NiMH rechargeables. My homebrew external battery pack for one of my memory card cameras weighs more than the digital camera. But most folks just put more batteries in while arguing to themselves that they are really saving much more on film and processing.

Note that I am not counting the cost of external strobe batteries here either, since that would be the same for film too. But on many digital consumer cameras, there isn't a provision for triggering an external strobe except by using the internal flash to trigger a slave photosensor on the bigger external flash. Most digital camera strobes have such low flash power that they can't really do much for lighting even at 6 to 10 feet. So you may end up with a much larger kit using a real strobe to light the eyes of subjects even ten feet away.

What if you elect to make digital prints of all your photos to 4x6" or 5x7" or whatever your photolab provides? You have to pay for the costs of paper and ink. If you want the highest quality photos, the proper papers cost around $1 per sheet. For four 4x6" prints, that's about 25 cents each - for high quality photo-grade papers. So if you are using the high quality paper, plus ink, plus factor in battery costs, the cost per 4x6" print is higher for digital prints than the cost of film and processing for mini-lab prints.

One nice feature of digital printing is that you can make pretty good quality 8x10" prints, even 11x14" and panoramics up to 11x48" on some color printers. You can, that is, if you start with a film image that is scanned into a digital file. Today's current 3 megapixel digital cameras may produce an acceptable 8x10" print on some shots, but few can do a full print (to the borders) on 11x14". You can find mail-order places offering 8x10" prints at modest costs (often just over $1 per print in bulk). I find it a bit paradoxical that the big savings with digital image processing and printing isn't feasible with current digital cameras, but rather only to those of us using both film and digital technology.

You can use regular paper or lower cost photo paper in many printers, but the quality will be less (and possibly less archival). Many printers and ink suppliers claim "archival" lives of up to 500 years for their products. However, real world experience has repeatedly shown disappointing archival qualities for many digital print materials and inks, especially if exposed to ultraviolet from fluorescent or sunlight sources. The fine print in some ads emphasizes that you must use the specified special papers to achieve archival quality.

One test by Ilford U.K. found that the cost of a high quality digital print on these archival papers with archival inks was virtually the same as a chemical print from film of the same size (i.e., over 2 GBP or approx. $3 US$). The lower cost papers and inks showed significant fading with sunlight exposure in less than six weeks in some cases. The archival inks on the wrong non-archival papers also showed noticeable losses within a year or so too. So you may have to keep reprinting those digital prints every year or so to avoid a bleached out or color shifted image from non-archival processing and materials.

Another significant cost is ink, which can run anywhere from a few cents per print on up (some printers require 3 or 4 ink cartridges at up to $30 retail a pop). Some printers doing maximum quality photo realistic prints may get only a dozen or so prints from a set of cartridges. So while it is possible to spend less on printer paper with some digital printers, you still have significant on-going costs for ink, paper, batteries, and other supplies.

What other supplies? How about all those CDROMs or zip disks that are storing your digital images? A 3 megapixel camera (at 16 million colors is 24 bits or 3 bytes per color pixel) works out to circa 9 megabytes of raw data per image. Even a 1 megapixel 24-bit color image is 3 megabytes. You can use lossy compression (e.g., JPEG) and greatly reduce these file sizes, but at the loss of already marginal image data and fidelity of the image. You will also need to backup your files on other media (e.g., negatives and prints are backups of each other). What I am suggesting here is that you have a significant cost in storage media which is often ignored by those claiming that digital camera costs are nearly zero.

Obsolescence - Digital's Hidden Cost Iceberg

Nobody wants a 640x480 web camera, or even a 1.2 megapixel digital camera, when you can get a 3 megapixel (usually 2.7 MP on chip) camera. As with older computers, the price drops precipitiously. That economic loss when upgrading to a new digital camera model every few years - new printer and new computer and new storage system and new software version of Photoshop... - hey, it all adds up.

Let's assume that your depreciation losses on upgrading your setup to a new digital camera and new printer are as suggested above. That works out to a $2,000 depreciation loss on hardware, software, and peripherals in two years, or roughly $1,000 per year. This figure is roughly the same as the running costs for the more active film amateur photographers shooting a few rolls of film per week. Our casual shooters burning a roll a month or $100 a year are spending rather less on their photography. So the vast majority of amateur photographers would be out less money if they are shooting film with film cameras with a much longer obsolescence period (e.g., ten years or $100 per year in camera obsolescence).

Speaking of obsolescence, don't forget that high power computer, disk drives, monitor, CDROM, backup tape drive, and color scanner. Now you need a CDROM burner, no make that a DVD reader, no, you really need this DVD burner and buggy software that goes with it. The syquest tape backup drive is out, you need a DAT backup tape system. The old 15 inch monitor is too small, you really need this 19 or 21 inch monitor. And your old color scanner is only 24 bits, don't you really need 30 or 32 bits? Don't forget to get the light table for it to scan film in too. You could use an Intel 486 for your internet email and office projects, but to run Photoshop with 128 megabytes you really need a Pentium II, or is it III or IV? If all this sounds familiar, it is probably because you too are on the digital express. As Alice in Wonderland said, you have to keep running to stay in place. Only with digital photography, you have to keep paying and paying!

Learning Curves

Speaking from my teaching experience, I can assure you that there is an arduous learning curve for non-geeks learning digital photography and computer technology. The cost of books and courses is also never mentioned by advocates of digital photography. You can often buy a nice coffee table sized photobook by a favorite pro photographer for the price of a thick and boring software book with CDROM.

The cost of image processing software, add-in packages for special effects, and other software packages is also not trivial. It is not unusual to pay more for computer software than for computer hardware, especially with some programs like Photoshop and the Adobe suite costing over $500 for a commercial copy. Don't forget to factor in all those digital photography and computer magazine subscriptions that you will be reading to learn the inside tips too.

And finally, every hour behind the computer monitor or reading a computer software book or manual is another hour you won't be spending taking pictures.

Color Reference

Beyond 16 Megasensor Chips?

I am suggesting that it may be hard to justify a costly 64 MP chip camera if you are just doing 11x14" prints. Many of us will be happy with 8x10" or 11x14" prints from a 16 MP digicam, just as we are today with minilab prints in this size. A square chip would provide 8192 pixels on each axis, or 4096 lines, yielding 500 mm of print with circa 8 lpmm print quality, or one meter (circa 39") of 4 lpmm print quality. But if you are just printing 8x10" or 11x14" prints, the higher density of the 64 MP chips may be overkill that won't show up in the prints. The human eye can't see or resolve the data past circa 8 lpmm, so the extra information might not be readily discernible?

Data Interpolation

Interpolation happens at a number of levels. The sensors on a typical chip are only able to measure levels of light, typically 8 bits or 0 to 255 levels of greyscale data. To generate a color picture element (pixel), we have to use at least three sensors, each of which is masked with a color filter to respond mainly to red, green, or blue light. The 8 bits of red, 8 bits of green, and 8 bits of blue data are used to create a 24 bit color value. In practice, we use a four element Bayer pattern of RGGB, partly because such a power of two array is easier to design, access, and build. Since the human eye is most sensitive to green light, the averaged green information provides the best and most pleasing image results.

However, some high resolution digital cameras are made using three separate chips. Each of these chips is masked with a different color filter, resulting in the required red, green, and blue color data.

While we currently use 24 bits of color data depth, other higher values are possible with lower noise and higher analog to digital converter bit depth (e.g., 30 bits of color data with a 10 bit A/D converter). We currently use 24 bits as the best compromise of cost and complexity against acceptable quality of the resulting millions of colors provided by 24 bit color depth. Color scanners have improved and increased their color bit depth from 24 bits to 30 and 32 bits and beyond, so digital cameras may follow suit in the future too.

Imagine a bathroom floor made of patterns of red, green, and blue tiles. The pattern is RGGB in a square or diamond shape. The software takes the observed 8 bits of red, blue, and (averaged) green data and generates a 24 bit color value for that square. That data point can be considered to be at the center of that square, and is a dimensionless point. But you can also realize that it represents the average intensity and color of the light falling on the light sensitive sensors in that grid of four sensors.

If it takes four sensors in the RGGB Bayer pattern to produce one color pixel, how does a 3 megasensor camera deliver 3 million pixels of color data? This process varys with different cameras, but in general, the camera uses the nearest available blocks of the required colors to interpolate a color data value at each point. So the four nearest red sensor cells to a blue block might be averaged to get an 8 bit estimated value for red at that point, and similarly for green. Now move on to the next sensor, say a red masked sensor, and repeat the averaging for the four nearest blue and green sensors around it. Keep going, stepping through the matrix of sensors.

One minor problem is that when you get close to the edges of the chip pattern, you don't have the required color data to project estimates for these edge sensors. For this reason, many chips are unable to provide quite as many pixels of color data as they have actual on-chip pixels. The larger the chip, the smaller the percentage of these lost data points. In some cases, the software tries to "mirror" or guess an interpolated color pixel value for the edges too, but the guess may not be very good.

A more interesting problem is that the average and maximum dimensions of these interpolated color pixels may be different from those computed using the close Bayer block pattern (RGGB). Blue sensors on a grid will have a pattern of four green and four red sensors in a box around them. Red sensors will have a pattern of four green and four blue sensors around them too. But what about the two green sensors next to each other? Ooops! The patterns must now be different. Depending on your approach, you will be using data from sensors farther away (e.g., to get four red and four blue values for averaging).

What happens to resolution if you are averaging in light from more distant sensors? In effect, you have generated an average color value from a larger area sensor, right? And a larger area sensor means lower resolution, given a fixed sensor density and chip size. So the chip resolution depends a bit on color and software algorithms used in interpolating these color values over the grid of the chip sensor array (excluding those "falling off the edge" values).

Effects of Chip Defects

Interpolation In Printing

In practice, few folks would find 16x20" color prints from a 3MP digicam to be of sufficient quality, and most would find 11x14" prints rather marginal.

One of the giveaways of low resolution (3 MP) digicam prints is their "creamy" texture. The texture is creamy because it is largely interpolated, with a series of interpolated values smoothing out the steps between actual data values from the 3 MP cameras (itself interpolated). Lots of people like this creamy smooth image effect, unless they have had experience with higher quality photo prints.

Sad to say, but few people nowadays have ever seen a high quality photographic print. In many minilabs, the enlargers are purposely defocused slightly to hide the effects of dust and scratches on your negatives. The low quality of many fast 800 ASA/ISO films also doesn't help. So the low quality of today's minilab prints has accustomed the public to lower quality photographic images, making the lower quality of digital prints seem as good and in many cases better than minilab photographic prints.

We are already seeing people rediscovering photography and high quality print making after seeing quality photographs such as the traveling exhibitions of Ansel Adams environmental prints or even a local camera club salon print competition. I suspect that one of the future benefits of film based print making will be precisely the ability to produce contrasty and detailed prints which extensive software interpolation and smoothing makes impossible in digital prints.

Best of Both Worlds

For many amateur users, modest cost scanners and color printers will make a nice combination with their existing computer systems. Personally, I think this trick will make 8x10" prints more accessible to many amateur photographers, including those who don't have access to a home darkroom.

One of the intriguing options here is to use the data from panoramic selections and print panoramic prints of any length (by setting printer software and using roll papers).

Another alternative is to have the film scanned on a drum scanner or other very high quality scanning device. The resulting digital image data files (at up to 600+ megabytes per image) dwarfs the image data provided by 3 to 5 megapixel digital cameras. Costs vary from $15 USD on up, with delivery via CDROM or over the Internet as popular options.

At a past Dallas Hasselblad University program, the discussion and interest on digital photography focused on the scanner based options. Perhaps this is just a reflection of the $50,000+ cost for a medium format digital back for Hasselblads from Dicomed and others? But many pros have found that adding a scanner and learning digital skills have opened up some new markets. But probably 3/4ths of pro photographers have yet to see enough advantages to begin to make the sizeable investments in time, money, and learning efforts to make the transition to digital. They may be wisely waiting for us amateur digital photographers to drive down the costs enough to make digital photography worth the real costs - including high depreciation rates.

Conclusions

Digital Optics vs. Lenses for Film

The biggest difference is that digital sensors have a low pass filter in front of them to help reduce problems with aliasing. This low pass filter is acting like a softening or fog filter on lenses used with film (which are also low pass filters of a sort). The result is to reduce the fine detail and high frequency contrast data from the lens to a rather lower range, typically 40-55 or so lpmm equivalent.

High Resolution and Fine Contrast Performance - Benefit or Curse?

So the first major difference between digital and film lenses lies in high frequency performance. With film, you need high resolution and fine contrast response lenses to take advantage of fine grained black and white and color slide films in particular. With digital sensors, such high frequency lens data creates a problem with aliasing. The ideal digital lens has a response curve matches the sensor requirements (to 50-55 lpmm equivalent). Then the digital lens response curve needs to drop to zero as rapidly as possible. Quoting from Schneider Optical's White Paper on Digital Optics:

Naturally, we are not interested in the reproduction of this false information and it would be ideal if the modulation transferred would suddenly drop to zero at the maximum line pair number. Unfortunately, this is not possible, either for the optics or the image sensor.

We must therefore pay attention that the total modulation transferred (from lens and image sensor) at the maximum line pair number Rmax = (2*p)-1 is sufficiently small, so that these disturbing patterns are of no consequence. Otherwise, it can happen that good optics with high modulation are judged to be worse than inferior optics with a lower modulation.

In other words, you want a digital lens that matches the response of the maximum frequency (e.g., 50-55 lpmm) response of the sensor, but then drops off as fast as possible. Most lenses designed for film are NOT designed for this requirement. What this means is that lenses designed for film, with extended high frequency response, are likely to perform less adequately than much cheaper lenses with lower modulation responses more compatible with the digital sensor requirements.

Some digital users will object that they can see improvements using high end lenses. No doubt they can, but this is more likely not due to the higher frequency resolution and high contrast response of these high end and high priced lenses. We can say that because that low pass filter in there is filtering out essentially all of the lens data above the low pass filter cutoff point (e.g., 50 or 55 lpmm).

Color Fringing Aberrations

But it is true that high dollar lenses get their higher resolution performance in part by correcting for a number of lens aberrations. For digital camera users, the two lens aberrations which cause color fringing (lateral and longitudinal chromatic aberrations) are especially noticeable and objectionable. So a lens which has excellent control of these and other lens aberrations may be a better performer than a lower cost lens which does not. Quoting Schneider Optics White Paper again:

The color fringes should be significantly smaller than a pixel size, which can only be achieved by using top quality lenses made with special glass having so-called apochromatic (high color correction) construction.

Fortunately, many lens designs don't have serious color fringing problems (e.g., normal lenses). But if you are doing telephoto shots of skittish birds, you may find that you need an APO or ED (for extra low dispersion glasses used) telephoto lens on your digital camera. Again, the more costly telephoto lenses feature such APO correction from specialty glasses, so they may perform better than lower cost zoom or fixed lenses that don't have APO glasses.

Telecentricity

Digital sensors are etched onto the surface of the sensor substrate with a series of layers. The result is that most designs have the light sensitive sensor element at the bottom of a "well" or hole etched in the flat sensor surface.

Now imagine light coming from an extreme angle out of a ultrawide angle lens. That light may be blocked by the walls of the sensor "well" and unable to reach the sensor. The solution to this problem is to redesign your lenses to project light at a restricted angle directly down onto the sensor, rather than at large oblique angles from the side.

This telecentricity factor is tele as in telephoto and reverse telephoto designs of wide angles. This reason explains why some very wide angle lenses not using a retrofocus design (e.g., hasselblad biogon 38mm) don't work well with some digital backs using sensors with deep well designs. These lenses send light at the sensor from extreme angles, some of which is blocked by the sensor well walls. Unfortunately, many of the best wide angle lenses are these non-retrofocus designs, which are most problematic with deep well sensor cameras.

Wide angle lenses which use the retrofocus design generally work well because they do present the light at a restricted angle directly to the sensor sites, and are not blocked by the side walls or well of the sensors.

Field Curvature

Field curvature is potentially a big problem with digital sensors, which are planar flat. The curved image field projected by the lens can produce a sharp central image by proper focusing, but the curved ends "lift off" the sharpest image zone away from the flat digital sensor.

In film, this factor accounts for much of the lower edge resolution seen in many lenses (though lens aberrations can also play a role). But with film, the emulsion is 20+ microns thick, sometimes much more with colored films using multiple emulsions. So this loss is masked by the thickness of the emulsion in part.

Macro lenses are specially designed to produce a very flat response with minimial field curvature. Such a flat response is a better match to the needs of the digital sensor. This is why the newly redesigned V series 40mm CFE lens for the Hasselblad system cameras is a very flat or macro lens design. This type of flat or macro lens response is very unusual in such a wide angle lens (equivalent to a 24mm or so on a 35mm SLR in horizontal coverage).

So lenses being designed for digital backs and larger digital sensors of the future have to take the problems of field curvature into account. But the vast majority of current 35mm and other film oriented optics are NOT macro lenses or flat field designs.

Coverage

Coverage is a well known issue to 35mm camera and lens owners looking at digital SLRs. Many DSLRs have less than full-frame (24x36mm) coverage on 35mm format DSLRs. The result is a cropping of the lens image to a smaller format (e.g., 16x24mm), often by factors of 1.6 or so. In other words, the full coverage of these 35mm or other lenses are not being used, just the center. So you are paying for coverage which you aren't using with such cameras. You are also carrying around lenses which are bigger and heavier than they need to be to provide such lesser coverage.

More importantly, if you are a wide angle fan(atic), you may be dismayed by this less than full frame sensor problem. You have to pay a lot more for a 14mm ultrawide angle lens for your 35mm SLR. But the 1.6X factor, for example, means that 14mm is acting as a 1.6*14mm or 22+mm lens equivalent. Ouch! If you prefer telephotos, this may be good news, unless you want the standard full frame coverage. Your 300mm telephoto now becomes a 480mm equivalent telephoto.

Naturally, you can buy some full frame sensor DSLRs, although often at a startlingly higher price. The larger sensors also have less noise, provide cleaner images, and other benefits. But as we have noted in this section, the use of lenses designed for 35mm film cameras with digital sensors may provide a less than optimal match.

Lens Speed

Lens speed is a potential win for digital cameras, though many of the current models don't take full advantage of digital sensor low light performance potentials. Similarly, the majority of digital lenses are used with autofocus systems, many of which require a considerable amount of light to function reliably (e.g., lenses faster than f/5.6).

Future digital sensor users may be able to use smaller and less costly lenses with their digital cameras. But today, most of the high quality APO telephoto lenses are also the faster and more costly pro models. Again, I suspect such lenses optimized for digital sensor use will be different because a smaller lens will be usable.

At the same time, the recent development of two electron release dyes (doubling the sensitivity of films from Kodak..) means that many films are now coming out in faster versions (e.g., velvia 100). Similarly, a ten-fold increase in film speed and linearity has been developed using formic acid couplers (e.g., Agfa). So both of these trends suggest to me that a new generation of smaller telephoto lenses may be in the future. Both film and digital sensors should have improved low light capabilities, so the need for large and costly fast lenses may be reduced. The key benefit from such lenses may well be the shallow depth of field they offer (see fast lenses pages).

Summary

Lenses optimized for digital sensors will be different from those for film camera use. They will have different MTF curves, with rapid falloff above some optimal sensor maximum response frequency. The digital lenses may well be macro lenses, with flat fields rather than the curved fields found on many film oriented lenses of today. Telephoto lenses and others may benefit from use of APO glasses to reduce chromatic aberrations to a minimal value relative to sensor sizes. Telecentricity requirements may force the use of retrofocus designs in all wide angles and most fast lenses. Telephoto lenses may be more moderate in maximum aperture. Tradeoffs in center resolution for better edge resolution response will also be used to produce better corner response. Some issues, like distortion, will be similar in both film and digital sensor lenses.

A system level issue is the lack of lens control electronics in most film camera lenses which fully match the needs of digital camera designers. Lenses designed for film autofocus cameras are not necessarily designed with the specific requirements of digital sensors in mind. For example, the aperture closure might be stepped, rather than continuous, or if continuous, non-linear rather than linear. Such problems have arisen with some lines (e.g., Nikon) and so a series of "improved" lenses were produced with the same optical designs often-times, but different aperture controls and related electronics.

Again, this will be a great excuse for the need for buying all new lenses for going digital. Conversely, digital and AF lenses are losing features that make them desirable for film camera use (e.g., dropping aperture rings on some Nikon AF lenses). So we may have a digital divide in the future, where one set of lenses is needed for optimal quality digital work, while another set is used with film cameras. From the sales and marketing point of view, this may be a plus, but less so for those of us who want to be both digital and film camera users.

What does this mean to us? I suggest that we will increasingly see advertisements reminding us that the latest generation of lenses designed for digital sensor (digital back..) use are superior in DSLR use to our old (and paid for) film camera lenses ;-).

The good news may be that digital lenses should potentially be less costly. Aberrations need only be corrected to the point where they are not visible in the relatively large digital sensors (versus sub-micron sized film grains). Lenses may not need to be as fast. While use of APO glasses may raise costs slightly, those lenses might be cheaper because they are slower aperture lenses. Lowering the high frequency lens response requirement will also greatly lower the cost of many lenses too. So there may be some surprises in the future!

Selected Postings

Date: Thu, 17 Oct 2002 
From: "Dr. Robert Young" [email protected]
To: [email protected]
Subject: Re: [HUG] Film vs Digital

I believe you are correct on this. I have developed a little "test" that I
have tried 2-3 times in the last 2 years. I have an image that came from a
high resolution scan of a 6x6 negative. I have it printed as 8x10 on
whatever printer I am considering (ink jet, color laser, etc) with proper
paper, ink, etc. I also had it printed at a digital service center,  and
printed directly off the negative. Then I let "someone else" look at them
with instructions not too be worried about slight color shifts (I know
those could be corrected with enough time in Photoshop). I simply want
their opinion of which is "sharper or clearer". So far, the  ink jets and
color lasers have never been selected as "the best".

The sad part is  I like to manipulate the image (burning, dodging, etc)
...I am a darkroom junkie at heart...and when you have to send every
version off to the service center for reproduction, it gets way too expensive.


 >Anyway, with the current state of the art and market, I think that inkjet
 >prints are largely a waste of time, being more expensive and of inferior
 >quality to true photo prints from a digital lab.




From camera fix mailing list:
Date: Fri, 18 Oct 2002 
From: "Mark Stuart" madfamily at bigpond.com
Subject: Re: Fungus

Interestingly, I just read a letter in Amateur Photographer about
fungus on CD's. The guy had stored photos on them thinking it was
more or less permanent, but apparently there is a fungus that thrives
on polycarbonate and aluminium! Looks just like some lens fungi. The
result was that they couldn't be read. They were stored in air
condititoning all their lives, too. Beware!

Mark



Date: Sat, 21 Jul 2001 

From: "Mxsmanic" [email protected]

Newsgroups: rec.photo.equipment.35mm

Subject: Re: Interesting News From Agfa




I thought Agfa was retiring products like APX 25 because it wanted to
concentrate on digital??  Just where is this company going?



The current digital market is one of mass-market gadgets, with the
exception of a few professional niche applications.  You have to be a
really big company to play in that arena.




"Meryl Arbing" [email protected] wrote 


> No future
> Agfa digital cameras and amateur scanners face the end
> 2001-07-05
> Are Agfa digital cameras and amateur scanners threatened with extinction?
> That's the way it looks at the moment at least. As Agfa let it be known at a
> press conference on 27 June, the company wants to gradually withdraw from
> the barely profitable Consumer Digital Imaging business segment.
> This business segment covers all digital products - principally digital
> cameras and scanners - for private customers. The department is part of  the
> greater Consumer Imaging division, which covers all products - analogue
> cameras and 35 mm or APS film included - for private the customer.
> Negotiations with the equity investment company Schroder Ventures on a
> possible sale of this greater business segment recently failed due to
> differing ideas about the takeover conditions. If it had succeeded, Agfa
> would also have got rid of its "unloved child" Consumer Digital Imaging. Now
> the negotiations have failed, Agfa wants to retain the Consumer Imaging
> sector but hive off the Consumer Digital Imaging sub-division. As it
> happens, its existing product range is to undergo minor improvements in the
> near future, but there's no longer any room for innovation. (yb)
>
> Like Agfa, Leica will find the digital market is "barely profitable".







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