Related Local Links:
Body Focus Errors in 8 of 10 New Cameras [2/2003]
Degradation Factors % (Erwin Puts)
Film Flatness Problems Pages
RF Focus Recompose Tables (tnks Jorgen Persson!) [8/2003]
Variations in Lenses
Medium Resolution Film (Tri-X) Shows Moderate Losses From Small Focus
Errors Wide Open
Source: Photography for the Serious Amateur, Eugen J.
Skudrzyk, 1971, p.254.
Low Resolution Film (Color Print) Shows Modest Losses From Small Focus
Errors Wide Open
Source: Photography for the Serious Amateur, Eugen J.
Skudrzyk, 1971, p.254.
For the standard lenses of 50mm focal length, a focusing error of 0.1mm
usually corresponds to a displacement of about 2 mm in the distance scale
on the lens mount [Ibid., p.255].
So if your focusing is off by a single millimeter on the lens scale, you
reduce your potential resolution to the 0.05mm curve as shown in these
charts. If you are off by 2mm, you should use the 0.1mm curves. Similarly,
focusing errors of 4mm and 8mm corresponding to the 0.2mm and 0.4mm curves
respectively. These values are for typical manual focus 50mm lenses for
35mm SLRs from the 1970s.
With today's autofocus lenses, you have even a shorter focusing arc. So
the same size focusing error on an autofocus lens with a short focusing
arc would correspond to an even larger error. So a 2 millimeter error on a
modern autofocus lens with shorter focusing arc might correspond to a 3mm
or even 4 millimeter error in focusing on the longer focusing throw lenses
of the 1970s. So one effect of autofocus
lenses is to increase the effect of focusing errors when used either
manually or in automatic modes.
The above charts should tell you many interesting things about typical lenses and film combinations. Lenses used with fast thin-emulsion films often have a peak resolution around f/8 (hence "f/8 and be there!" rule of thumb). Beyond f/8 to f/32, diffraction limits lens performance. Below f/5.6, many lenses are limited by aberrations. Wide open stops also project broader cones of light into the film emulsion, further reducing the resulting lens-film resolution. And as the top chart shows, errors in focusing can also contribute significantly to losses in resolution when good lenses and thin emulsion, high resolution films are used.
The second chart highlights Skudrzyk's observation that a thicker emulsion from using a faster speed film (like Tri-X) reduces the potential resolution we can achieve. The lens still performs well around its middle apertures (f/8). But the cone of light from the lens spreads out more in the thicker emulsion, reducing the achievable resolution (i.e., thicker lines mean lower resolution levels). Focus errors still have an adverse effect, but not as pronounced as when using thinner emulsion films as in the top curve.
The last chart should be truly discouraging for those folks shooting color print film. Color film has multiple emulsions, usually at least three layers corresponding to each primary color (red, green, blue). Color film also has various color filter layers which add to the thickness of color film too. The resulting drop in resolution is severe when compared to single layer black and white emulsions. Errors in focusing are masked by thicker color emulsions which limit the resolutions you can achieve. Amazingly, even stopping down beyond f/16 doesn't have much impact. The resolution from thicker color film is already so poor that diffraction doesn't have much of a chance to be a limiting factor.
I should add that these tests were made with 1970s era color film. So unless you are using up old stocks of dated color film, chances are good that you will have much improved color film resolution with today's films. In general, your experience with modern color film will be rather more like the center chart above (I hope!). However, even the best color films of today still require at least three emulsion layers plus filter layerings, making them much thicker than the single emulsion black and white films.
Not all lenses will perform as these typical normal lenses on 35mm SLRs. So it behooves you to actually test out your lenses with your preferred film stocks to see exactly how they behave at different stops. But our next section highlights Skudrzyk's other discovery - how much small errors in focusing cost us when using higher resolution film and lenses. Let us look at a way to measure this on your own equipment next.
In just a few minutes, you should find that you have a number of marks on
your lens. You probably won't get the lens to focus at exactly the
same point every time. You should instead have a distribution of focusing
points along the lens barrel. Observing the size in millimeters of
your focusing range, and comparing to the above observations and
test charts, should be quite sobering.
If you would like a deluxe and documented home focusing test, start with a
bit of graph paper with millimeter grid. Trim a sliver of graph paper a
few boxes wide so it can be taped or glued with removable rubber cement
onto the lens barrel. Now make the above focusing test again. Measure each
focusing point along the millimeter scale as accurately as you can. If you
like, you can use a tape measure to get the exact focusing distance to the
wall for comparison. Note that we are using a flat wall parallel to your
position so the focusing subject is absolutely flat. Check where this
distance as marked on the lens falls on your millimeter scale setup.
Hopefully, your average focusing point will be close to this marked
focusing distance on the lens!
Now using the graph paper, you can determine how far each of your focusing
efforts ranged above or below the measured focusing distance (and
corresponding mark on the lens). Simply write down the relative position
along the graph paper of each focusing measurement (e.g., from some end
point). You can take an average of your ten or twenty readings, including
the outlying data points. Using this average focusing point, calculate the
offsets above and below this average point for each of the marked data
points. You should end up with a table of offsets for each focusing
effort. We will use the average point as an estimator of the true focusing
point, which is probably reasonably statistically accurate for
twenty or more observations.
So where is the horror in all this? Simply recall that a focusing error of
2 millimeters on a typical 35mm SLR manually focusing 50mm normal lens
corresponds to roughly 0.1mm or a 100 micron shift in focusing position.
If you are using high resolution (slow) film at f/4, your potential
resolution of well over 100 lpmm has fallen to circa 60 lpmm. Wide open at
f/2, you will be lucky to hit 40 lpmm, while more accurate focusing would
have produced circa 120 lpmm. Are you horrified yet?
n = 1.2/r = 2.4 * (F/delta-i) [Ibid., p. 252]
where:
n= resolution
r=radius of disc of confusion
F= F/stop
delta-i = focusing error
(assuming focusing error is only source of image deterioration)
As an example, a focusing error of 2mm on a typical lens scale corresponds
to 0.1mm offset in the focus plane from the ideal focal plane. At f/1, the
formula gives 24 lines/mm as a resolution estimate, versus 240 lines/mm at
f/10 [Ibid., p.252]. What I find interesting about this formula
and estimator is that it suggests how large a role focusing errors and
depth of focus shifts play in limiting the resolution achievable with fast
lenses. While lens aberrations are usually blamed (see fast lenses page), or buckling
and unflat film, it may be that difficulties and errors in focusing
wide open play a larger role than is often imagined in limiting fast lens
performance at the wider stops.
| f-stop | 0.05mm | 0.1mm | 0.2mm | 0.4mm |
|---|---|---|---|---|
| 1 | 48 | 24 | 12 | 6 |
| 1.2 | 58 | 29 | 14 | 7 |
| 1.4 | 67 | 34 | 17 | 8 |
| 2 | 96 | 48 | 24 | 12 |
| 2.8 | 134 | 67 | 34 | 17 |
| 4 | 192 | 96 | 48 | 24 |
| 5.6 | 269 | 134 | 67 | 34 |
Or for a graphical view, highlighting the rapid falloff of resolution with
focusing errors and f/stops, we derive the chart below. Again, I
caution that the underlying formula is only considering the effects
of focus errors, and not factoring in the effects of lens
aberrations. Below 0.1mm, lens aberrations wide open can easily
exceed the impact of lens focusing errors (e.g., broadening the
circle of confusion by the aberration's effects). The key point in
showing this chart is to highlight that the effect of approaching
the point of optimal focusing is not linear. Rather, accurate
focusing results in much faster improvements and increases in lens
resolution figures than might be expected. Conversely, unaccurate
focusing by even small amounts means the optimal lens resolution
will not be achieved at these wider stops, especially with faster
lenses.
This chart also helps explain why fast lenses have an extra degree of
"snap" when focusing. Users note that the subjects often seem to simply
jump into sharp focus. Now you know why that's true. As you approach the
point of optimal focusing, the resolution improves significantly and
non-linearly as you get closer and closer to the optimal focus
point.
The situation for high resolution, thin films is rather different. Quoting
Skudrzyk, The curves of Figure 91 [viz., top 3 charts] lead to a very
important conclusion: they show that focusing is very critical for
high-resolution, thin-emulsion films. Since the resolution of such a film
decreases greatly if the F-stop is increased above F/4, the loss in
resolution because of a focusing error can never be compensated by
stopping the lens down...
[Ibid., p. 255].
So let us recap our situation. If you are using fast black and white
films like Tri-X, or low resolution color print films (as 94% of amateur
photographers do), then you won't run into focusing error issues very much
even when shooting wide open. But if you are shooting high resolution films with fast lenses at
apertures below f/8, then focusing accuracy can become an issue.
I have previously related how hard it is to break the 50 lpmm barrier. Some lenses are subject to
focusing shifts when you stop down. Other lenses have color correction
issues (e.g., telephotos) which can result in multiple focusing points for
different colors of light. But even if these problems are under control,
it is hard to get the most out of your fast lenses and high resolution
films. My own efforts include use of chimney finders with high
magnification factors (e.g., 3X to 5X) on my medium format cameras. On
35mm, a popup 2X magnifier can provide an extra degree of focusing
accuracy, particularly with macrophotography.
For many photographers, their real world subjects are three dimensional,
so some part of the subject will hopefully be in the plane of optimal
focus and highest resolution. The problem comes in trying to repeatedly
place that plane where you want it (e.g., on subject's eyes) even under
dim lighting conditions mandating wide open exposures. On some autofocus
cameras, you can select a "focus bracketing" option that varys the focus
slightly with each of three shots, hopefully getting a sharp photo on the
desired subject by this programmed technique. You can do likewise with
your critical shots, especially if the subject is flat and the lighting is
marginal.
But the eye cannot focus accurately using ground glass. Ground glass can
resolve circa 100 to 200 lines per millimeter. The ground glass image on a
typical medium or large format setup may be magnified five-fold (e.g.,
chimney magnifier, loupe). The eye, with the magnifier, then resolves 2
to 3.5 times fewer lines than the ground glass or a fine-grain film. This
means that the eye cannot discern the point of optimum focus but can only
see certain limits on each side of the point of optimum focus where the
image becomes blurred. By finding these two limits and by placing the
focusing position between them, a focusing accuracy corresponding to a
ground-glass resolution of 120 lines/mm is obtainable with an f/2, f=50mm
lens in about five out of six trials [Ibid., pp. 271-2].
I call this trick "rocking focus", meaning you are rocking the lens
back and forth until you reach some compromise central focusing point
between two roughly equally out-of-focus points. On a typical fast lens,
this back and forth distance may be only a a few millimeters as I narrow
down the focusing spread before selecting the final hopefully central
focusing point.
You can get some feeling for the effect of maximum focus accuracy by
photographing a lens test chart at various focusing positions. Focus at
0.5mm or less distances along the lens focusing scale, and shoot the chart
(preferably using strobe lighting). The result should be a series of
charts with varying resolution. Every now and again you will get an
unusually high value due to nearly perfect focusing. Naturally, you have
to use a very high resolution thin emulsion film to observe these
effects.
| Focus carefully (and manually, all tests we have encountered to date show that careful manual focus is best, followed by autofocus, followed by sloppy or hurried manual focus). Oh, and avoid zooms except for a very few of the very best. |
|---|
The chart below shows one of these possible outlying
data points, the reported 80 lpmm center performance at f/4. Is this a
typo in the original data or testing, which should be 70 lpmm? Or is it a
result of nearly perfect focusing? If lens resolution testing used
statistical techniques of multiple tests of each data point, then we would
not be left in this quandry. But as things currently stand, you are left
wondering by the data presented above if many lenses are under-rated due
to failure to reach an optimal focusing position.
| f/stop | Center lpmm | Edge lpmm |
|---|---|---|
| 2 | 56 | 35 |
| 2.8 | 63 | 45 |
| 4 | 80 | 50 |
| 5.6 | 70 | 56 |
| 8 | 70 | 56 |
| 11 | 63 | 56 |
| 16 | 63 | 50 |
This trick should sound familiar. Recall Skudrzyk's statement The eye,
with the magnifier, then resolves 2 to 3.5
times fewer lines than the ground glass or a fine-grain film. This means
that the eye cannot discern the point of optimum focus but can only see
certain limits on each side of the point of optimum focus where the image
becomes blurred. By finding these two limits and by placing the focusing
position between them, a focusing accuracy corresponding to a ground-glass
resolution of 120 lines/mm is obtainable with an f/2, f=50mm lens in
about five out of six trials[Ibid., pp. 271-2].
In other words, the Messraster is designed to overcome the eye's
limitations of accurate focusing by limiting the focusing error to a few
tens of microns (i.e., 0.01mm+). The subject literally jumps into focus,
according to users and reviewers, making precise focusing easy. But
unfortunately, using a Messraster style screen would require individual
calibration of each camera to match the screen precisely to the camera's
film focal plane. Other problems such as film
buckling would also become limiting, especially with some popular and
expensive rollfilm medium format SLR cameras. For many users not using
fast lenses nor fine grained film, the costs would not justify the results
(or they would be using finer grain film etc.).
Notes:
From Modern Photography, November 1980, SLR Notebook, H. Keppler, p. 72
Finding the sharpest point takes practice. Rotate the focusing ring,
quickly at first, back and forth across the plane of sharpness, gradually
narrowing the swing until you get close to the correct sharpness plane.
Then turn it very slowly. it will take some time until you are certain
when the subject is sharp.
Related Pages:
"If you allow a pinpoint of light to be recorded as a blur of .01mm (1/100mm) diameter, and work at an aperture of f/4, then the allowable depth of focus (at the film plane is only .04mm (You may take only half of the full allowance when working at infinity)*. This tolerance includes all factors which can influence the final image focus, such as the natural bend of the film, tightness of the lens mounting flange or thread, accuracy of the infinity stop on the barrel, and accuracy of the distance of the lens flange and the film plane as determined by the camera body." [Source: Bennett Sherman and Al Gordon, "How Sharp Can You Get", Modern Photography, October 1978, p. 170].
For related discussion on breaking the 100 lpmm quality barrier with 50mm normal lenses, see our breaking the limits of photographic quality pages.
[Ed. note: * I believe because DOf is split symmetrically at infinity and at macro levels, otherwise, usually stated as 1/3rd versus 2/3rds split at intermediate distances].
From Contax Mailing List:
From: "Philip Coghlan" <[email protected]>
Subject: Re: [Contax] re:depth of focus
Date: Wed, 22 Aug 2001
-----Original Message-----
From: Austin Franklin
>
>There is a table there that has numbers, but I didn't see the formula used
>to calculate the table...and it was only calculated at infinity. I am
>curious what the changes to the table would be at nearer focuses.
>
I'm not sure if this is what you want, but these formulae are given in the
Manual Of Photography:
For general photography, where the magnification is small:
t = 2CN
where t = depth of focus, C = circle of confusion, and N = relative
aperture (f no)
So for a CoC of .02mm, and f2, depth of focus = .08mm
Where magnification is significant:
t = 2CN(1 + m), where m = magnification.
Alternatively:
t = (2CNv) / f
where v = image distance & f = focal length.
Regards Philip C
from contax mailing list: From: Michael Londarenko mikel@[deleted] Subject: RE: [Contax] RE: The Digital Way. One more (last?) time Date: Fri, 11 Jan 2002 [snip] Ok, ask Zeiss. Their lenses for motion-picture cameras (ARRI) can easily resolve 200 lp/mm in center. But the actual frequency on film almost never exceeds 40 lp/mm. With extremely accurate testing conditions labs can get no more than 80 lp/mm on film. Film manufacturer published data is different story. At 40lp/mm, Planar 50/1.4 at f/4 offers about 80% MTF in the center. With focus error of only 50 microns in image space this value drops to 60%. With focus error of 100 microns it drops to 20%! Now think about what is it going to do with the actual image quality and resolution. Numbers like 200lp/mm are non-achievable with film photography. [snip]
[Ed. note: variations in film backs and film holders may also be an issue in LF?] From: [email protected] (Richard Knoppow) Newsgroups: rec.photo.equipment.large-format Subject: Re: Depth Micrometer Purchase? Date: Sat, 25 May 2002 .... For reference the ISO-ANSI Standards for film plane placement in sheet film cameras is: Size Location Tolerance + or- 4x5 0.197 0.007 5x7 0.228 0.010 8x10 0.260 0.016 Dimensions in inches. This is the distance from the reference surface for the film holder to the film surface. It is the distance from the reference surface to the ground glass. For checking the film in a film holder it must have film in it. Most sheet film is 0.007 inch thick. --- Richard Knoppow Los Angeles, CA, USA. [email protected]
Date: Mon, 27 May 2002 From: "Greg" [email protected] Newsgroups: rec.photo.equipment.large-format Subject: Re: Depth Micrometer Purchase? Great subject. Never seen it up before , reckoned no one cared. Sinar reckon on .3 mm that's about 12 thou. Richard quotes 7 thou. Truth is if your limits are too high there would be nothing good enough to pass. Some of my slides are ok, i.e. < .3 mm. Most are right on the border. I've only ever taken one back for exchange and it was way out. The dealer was quite fascinated by the testing demo. I feel he thought I was a bit nuts. I use a motor cycle TDC dial gauge. It's small and comes with a fitting enabling it to be screwed into the spark plug hole. I use a straight piece of aluminium twice the width of the back.and the device is threaded into the middle. First check the GG and make shure that it's flat to the back and adjust (zero)the gauge. Then with a sacrificial film loaded one can check to see if the film holder is parallel to the GG. and the same depth i.e. from the lens, by sliding the alum. across the face of the film. I allow up to .3mm What ever you do don't check your Polaroid back. The shock might kill you. And Polaroid just don't give a dam. Still its better being alive taking blurred pictures that dead and not taking any. My new back was something like 40 thou. out, yes one whole mm!! On inspection of other backs at the shop including the rental ones I found mine to be the least out.?? One was over 70 thou out of Parallel to the GG and about 20 thou. further from the lens to boot. The Polaroid rep wondered what I was on about. Now if I was to buy a digital thingamabob and join the screaming headlong rush to consumerism my problem would cease to exist? Was it Einstein that said problems can neither be created nor destroyed only shifted from one less than perfect albeit satisfactory long term photographic system to an extremely short term plasticised version. -- - -- Greg. -- [email protected]
[Ed. note: a good reminder that labs and printing can easily mess up as well ;-( ] From hasselblad mailing list: Date: Tue, 23 Jul 2002 From: Ken Martin [email protected] Subject: Re: [HUG] Auto Focus or Focus Confirmation Beau: I assume that you used a tripod and cable release and locked up the mirror. If you did not the problem could be technique. The only way to judge the performance of a camera lens is to look at the transparency or negative that it produced and not the final print. As you suspect, something could have gone wrong in the steps from the positive/negative to the print. I have had many prints come back from a professional lab that are soft that when reprinted were tack sharp. Also there is a danger in judging a lens using a distant landscape. Haze and dust in the air can make the photo look soft when the camera and lens did its job. All lens perform better at certain f/stops than at others with the intermediate f/stop being the best. That is where the old adage "f 8 and your there" came from. I have found that only on very large enlargements is there a significant difference in sharpness with my Hasselblad Zeiss lens from an f 8 to a f 22. You may want to shoot a roll of transparencies at different f stops using a high resolution film like Fugi Velvia and then examine them on a light table with high power loupe. That should tell you more about you lens. Hope this helps. Ken
From hasselblad mailing list: Date: Wed, 25 Sep 2002 From: Jim Brick [email protected] Subject: Re: [HUG] Hasselblad H1 AF Tom Just Olsen wrote: >I see not the big advantage in AF anymore (after >having switched from Canon FD to EOS). And certainly not on any MF >equipment. It makes it all to heavy. Nor is AF reliable, - a very >little talked of issue. With AF; still you have to check thoroughly that >the AF has focused on the right thing. > >Tom Just Olsen AF lenses, when used in AF mode always have less resolution (lp/mm) than the same lenses manually focused. This is because AF cannot achieve critical focus. If it does, it is just luck. Focus is typically achieved by digitizing the analog signal from a CCD sensor and creating a histogram from the digitized result based on contrast differences. As the focus gets closer, the histogram center spike gets larger. When it passes through and starts to get smaller, then the AF system backs up the number of steps it went past the high point. The problem is that there is considerable slop in the focusing that produces the high point. A range + and - from the critical focus point that allows electronic focus to think it is there. There is no solution for this. Other than using your eye and brain to do manual critical focusing. And... AF frequently cannot focus on the very thing that you want to focus on. It cannot see it. There has to be a contrast line between what you want to focus on and the surrounding area. But if this line runs in the opposite direction of the how the CCD focus sensor is positioned, you are out of luck again. Many of the latest AF systems have attempted to fix these problems. Orientation is the easiest to fix. Having little or no contrast between the focus point and the surrounding area is almost impossible to fix. I played with a Contax MF AF camera a while back. I tried to focus on a Plexiglas magazine holder, from the side, on the counter at KSP. It was holding View Camera magazine. The AF hunted forever. We all had a good laugh. It took about 1/2 second to manually focus on the edge of the magazine holder. I believe that the H1 will sell very well. It has all of the features that wedding (and similar) photographers really want. Auto everything. Auto exposure, auto flash and auto flash fill, auto focus, 32 frames per roll, interchangeable backs with an LCD that tells what's in the back and how much is left, among other things, motor, hand grip, remote, seamless integration of digital, etc, etc, etc... My dealer, who sells more Hasselblad equipment west of the Rockies than anyone except Samy's, say that they can sell the h*** out of these things. And I believe them. Me... Im a square. I have enough trouble dealing with my rectangular 35mm & 4x5! Jim
From: [email protected] (FLEXARET2) Newsgroups: rec.photo.equipment.medium-format Date: 07 Nov 2002 Subject: Tweaking Focus on Med Format What amazes me is the number of medium format SLRs and TLRs I have examined over the years where the finder screen focus is "slightly off" in comparison to the film plane. Some manufacturers and repair technicians use various forms of flexible media (ie. mirror mylar) in the film plane to set this focus through instruments projecting a pattern through the taking lens and examining the reflection, based of their assumption that film is not truly flat in the aperture/gate. This is an approximation, as no two types of film, rolls of film or exposures on the same roll of film can possibly display the same exact amount of "unflatness", if there is any to speak of. I have examined SLR cameras without the lens on, and shutter open, and test roll in the camera and found many camera brands to have good film flatness, which implies to me that a groundglass at the film plane is far better to set the alignment with rather than some flexible media. After expoxying a loupe to the shiny side of a piece of 6x6cm groundglass I use this on the aperture rails to compare to the finder focusing screen setting and then set this, if it is slightly off, to match the focus on the image on the groundglass. Since I have always gotten sharper photos on test films after this adjustment is made, I thoroughly believe many cameras need this adjustment to get the maximum sharpness out of their fine lenses, without needing to stop down to do so. - Sam Sherman
From: [email protected] (Hemi4268) Newsgroups: rec.photo.equipment.medium-format Date: 05 Feb 2003 Subject: Re: Q: Mamiya RZ lens resolution? >Try to find out the criteria used by NASA when they were shopping for >camera systems to use in space - should give you a starting point. > I can't say anything about NASA but since I was chief of the CIA spy camera program from 1985 to 1996 now retired I might have a few insights. First, just about all name brand lenses have about the same resolution although for special applications, suppliers would allow us to source select. This means we would take delivery of say ten 180mm lenses and select 5 sending the rest back. Biggest issue was with the camera bodies. Only about 2 out of 10 camera bodies focused well enough to meet specs. This means the other 8 camera bodies had mirrors that were not in sync with true focus. In other words, when you focused on something at 20 ft you were actually focused at 16 or maybe 25 ft. This is a full 100 micron focus error that won't really clear up until stopped down to at least f11. Just about all image quality problems can be traced to the camera bodies. It's very rare indeed to trace image quality problems to a lens from a major manufacture. Larry
From camerafix mailing list: Date: Mon, 14 Oct 2002 From: "Kelvin" [email protected] Subject: Re: Mamiya 645 hi, Have you checked the various hooks / catches to ensure none were deformed in the impact? Maybe retightening one of them would help. That aside ... depending on the nature of the impact, I am thinking ... is there a need to worry about the alignment of the working distances in camera body? Years ago when I dropped my ETRSi with a 50/2.8 mounted.... the camera had to go back to Japan. The impact was so bad that Bronica replaced the entire camera casing and the focus helicoid in the lens.... the impact had misaligned everything.
[Ed. note: an error in focusing can be expected in many new in box (let alone used) cameras, per Larry's posting] From: [email protected] (Hemi4268) Newsgroups: rec.photo.equipment.medium-format Date: 29 Jun 2003 Subject: Re: Lenses - yesterday, today and tomorrow >Aren't the 10 I keep "store bought?" Just another method "source select" to try to play the averages to find the best cameras. Actual error would be on a normal distribution curve. Some store bought cameras would have no error and some would have slight error and again some would have even more error. Spec usually calls for less then 100 microns of error or about 2 feet at 20 ft. So a camera that actually focuses at 18 ft when the ground glass says 20 ft is in spec. A group of 20 store purchased cameras would contain at least 14 cameras that have less the 50 microns or error. meaning that true focus is within 1 ft at 20 ft. About 3 or 4 cameras out of the 20 would be perfect, at least 10 additional cameras would be in spec. You might see 5 slightly out of spec and maybe 1 or 2 totally out of spec with more then 200 microns of error. Larry
From: [email protected] (Hemi4268) Newsgroups: rec.photo.equipment.medium-format Date: 28 Jun 2003 Subject: Re: Lenses - yesterday, today and tomorrow >What other kinds of cameras are there, besides store-purchased? And why >does purchasing a camera in a store make a difference? Nikon for example will custom make a standard camera for you. Cost is about twice the store bought price. This is usually done for say a government purchase of 10 cameras. You would want to do this to make sure the ground glass is in the exact same position as the negative. Another type of purchase is called a source selection. Say 20 Nikons are purchased from B&H with the understanding that 10 will be selected and the rest sent back. >Not any $200 lens I've ever seen. Not even any $2000 lens. You really need a good portable optical bench to do these tests. All tests are center resolution high contrast in noon summer sun. Yes, doing it in a cold winter rain with fog will lower the results. Larry
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