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Close Focusing Distances for MF Lenses
Close Focusing Distance - A Critical Feature?
Tale of 3 105mm Lenses (Series E zoom wins?)

Discussion

The short answer is that close focusing factors are one way to separate the great lenses from the pretty good ones. Many of the professional OEM lenses have very good close focusing distances, often half of the limit of lower cost import or third party lenses. Some of the third party lenses which reached cult status, such as the early Vivitar Series I, were renowned for their close focusing distances.

Besides close focusing distance, the quality of the image when focused close is also a supreme challenge to many lens designers. Generally, the resolution and other lens quality factors drop significantly when you get near the close focus limits. For this reason, Nikon introduced CRC or close range correction elements in some of their innovative pro lenses such as the 24mm f/2.8 with CRC. By floating some optical elements in the lens when getting into the close focusing range (using cams..), the lens designers were able to significantly improve closeup performance.

Today, you will find many professional lenses achieve both closer focusing limits and higher resolutions and quality at these near limits by using these floating element designs. In most cases, the designer automatically cams the floating lens elements, so as you focus closer the floating elements track to provide such effects. In a few cases, such as the Zeiss updated designs for various Hasselblad lenses (40mm T*..), these close range correction floating elements have to be dialed in by the user using a separate control ring. I find it odd that a $400 US lens does this automatically, but a $4,000 lens requires you to do it manually, but at least the feature is there.

Shorter Zoom Lenses versus Close Focusing

Lenses at infinity are usually at their closest point to the camera film plane. Focusing on a closer subject makes you rack the lens out, using the helical focusing ring. To get closer, you just have to keep going out and out and farther out. In most cases, your lens has to have a long helical focusing throw or distance to get close focusing. For similar reasons, a lens head on a bellows that gets farther from the camera body produces closer and closer macro focusing.

So what happens when you have a lens that is only three inches long instead of six inches long? The distance you can rack out the helical focusing tube on the lens is much shorter. As a result, the close focusing distance of most of those shorter, lighter super zooms is rather limited. Where an older longer superzoom might close focus to perhaps three feet, the newer shorter superzooms usually focus to only six feet or so.

When is a 28-200mm Tamron Zoom really only a 35-110mm?

This Tamron lens is actually an example of a new design lens which uses complex internal optical tricks to enable this relatively short lens to provide a more useful degree of close focusing capabilities. Many internally focused super zooms won't focus as close, but will also experience a contraction in their focal lengths at distances other than infinity.

The same problem afflicts many macro fixed focal length lenses using internal focusing. So your 100mm macro IF lens suddenly starts acting like a 75mm or 80mm focal length lens when used up close. For many nature subjects which are either dangerous (e.g., rattlesnakes) or easily frightened (e.g., bugs), you want the longest macro focal length you can afford. A 100mm macro lens should in theory let you get twice as far away as a 50mm macro lens, while a 200mm macro lens should let you double the distance again.

Unfortunately, 200mm and longer macro lenses are quite a bit more costly and rarer than 100mm macro lenses. The shorter 50-55mm macro lenses are often the cheapest in most OEM lines, so many of us start out with them as our first macrolens. So when you spend major sums to get a 100mm macro lens, you are hoping and expecting it will really turn out to be a true 100mm macro lens. You are buying it so you can get that longer subject to lens distance for safety and usability. But many longer macro lenses (especially with internal focusing) turn out to shift their focal length towards a shorter focal length, reducing the effective lens to subject distance to little more than the 50mm macro lens provides. Drat!

One solution is to get a true macro lens without any internal focusing elements. For example, I have a Novoflex 105mm f/4 autodiaphragm bellows mount (nikon) macrolens. This bellows macro lens provides the expected subject to lens distance, unlike one of my compact third party 100mm macro lenses with internal focusing. Are you planning on buying a longer focal length macro lens in the hope that it will provide some more distance between your camera and lens and the (skittish) subject? If so, you should carefully check the actual front of the lens to subject distances at various closeup distances. Be prepared for some surprises!

Here is a table of 100mm macro lenses in which this critical lens to subject distance varied from a mere 12 inches to circa 17 inches! Now notice that the 200mm f/4 Nikkor macro lens only provides 19.4" of stand-off distance. Some 100mm macro lenses such as Pentax already provide almost 17 inches of stand-off distance. Surprise!

 Many of us make major investments in buying a second macrolens in the 200mm range in the hopes of getting circa double the standoff distance from those skittish or dangerous subjects.  You would be understandably upset to discover that your 200mm macro lens only provides an additional few inches of standoff, rather than the extra foot or so you expected. So before you buy that macro lens, check carefully into these issues to avoid unpleasant surprises! ( table courtesy of posting by C Lanier Benkard):

          Lens                   Lens to Subject Distance at 1:1
          -------------------    ------------------------------- 
          Zeiss 60/2.8 Macro     10"
          Tamron 90/2.8 Macro    11.4"
          Canon 100/2.8 Macro    12"
          Nikon 105/2.8 Macro    12"
          Pentax 100/2.8 Macro   12"
          Sigma 105/2.8 Macro    12.3"
          Minolta 100/2.8 Macro  14.4"
          Zeiss 100/2.8 Macro    16"
          Pentax 100/3.5 Macro   16.9"
          Canon 180/3.5 Macro    18.8"
          Nikon 200/4 Macro      19.4" 

As another example, the Sigma 28-105mm f/4-f/5.6 zoom at 105mm provides a 1:4 image at 480mm (19 inches), while the Vivitar 105mm Series I macro provides a 1:4 image at 660mm (or 26 inches). [Source: SLR, H. Keppler, P. 26, July 1996 Popular Photography]. Here again, you would suspect some focal length compression in the effective zoom lens focal length at the marked 105mm setting.

Teleconverters and Close Focusing

This trick also works on other lenses. Suppose you have a 300mm f/2.8 pro lens, and put on a 1.4X teleconverter? The resulting 420mm f/4 equivalent lens will retain the shorter close focusing distance typical of the 300mm lens, rather than the longer close focusing distance usually found on 400mm lenses. Similarly, a 2X teleconverter would yield a 600mm f/5.6 lens with the same shorter close focusing distance of the 300mm lens. Using a longer telephoto at the same close distance produces much larger image sizes on film. What you have done is increase image magnification by using the longer telephoto equivalent lens at the same closer focusing distance of the shorter lens. Make sense? So you get a much bigger image on film with the same subject distances.

Can an Economy Zoom Beat a f/2.8 Pro Zoom Up Close?

One interesting and surprising example in lens design tradeoffs is the Sigma 28-105mm f/2.8-4 zoom lens. What if you compared this variable aperture economy zoom against the Tamron 28-105mm f/2.8 fixed aperture pro zoom? At infinity, the Tamron f/2.8 pro zoom is sharper by a full grade (e.g., SQF of 96.3 versus Sigma SQF of 84.5 at 105mm and f/5.6 for 11x14 prints [Source: SLR, H. Keppler, p. 17, March 1998 Popular Photography].

What if you adjust these zoom lenses to 105mm setting and focus to ten feet, to take a portrait? The Sigma and Tamron now reverse positions. The variable aperture prosumer Sigma 28-105mm zoom is now a full grade sharper than the pro quality Tamron 28-105 fixed aperture f/2.8 zoom. Surprise!

What's going on here? The Sigma lens designers evidently decided that the 105mm setting would most likely be used for portraiture at distances closer to ten feet than for infinity shots. So they optimized the lens for this expected and reasonable real-world use. But the Sigma lens was penalized against the Tamron lens on the much quoted Popular Photography lens tests, which are performed at infinity distances only. But using the Tamron 28-105 f/2.8 pro zoom at ten feet produced poorer results than the variable aperture Sigma prosumer zoom.

(see A Tale of 3 Lenses)

Lens tests are prejudiced against macro lenses. As noted above, most magazine lens tests are done only at infinity focus settings. For lens testing, the "infinity" distances used are usually at least 100 times or more the focal length. So when you get closer than infinity for tasks like portraiture or closeup macrophotography, the infinity lens tests don't really tell you how the lens will really perform.

Now you know why so many PHOTODO lens tests give such modest or low numbers to many macrolenses. These lenses are optimized for use at closeup distances, not infinity. When measured at infinity, and compared to lenses optimized for infinity, macro-lenses typically don't do so well.

Now you can understand why a low cost Series E (for "economy") 70-150mm f/3.5 zoom did so well against the 105mm f/2.8 macro nikkor lens (and classic 105mm f/2.5 portrait lens). These lens tests were at distances where the cheapy zoom lens was optimized (1:50 subject size). The macro lenses really are better when used closeup than even a good modest range 70-150mm f/3.5 consumer zoom like the Nikon one used here. But this reality is obscured by testing these lenses at infinity.

So if you need a lens for portraiture work, select one optimized for that distance (or a zoom like the 28-105mm Sigma cited above). If you need a macro lens for closeup work, pick a lens optimized for that distance. If you intend to use a lens at infinity, the lens tests will provide some insights. But you should still test the lens in your hand to see how it actually works, rather than rely on someone else's lens test on another lens sample. On the bright side, you now have an excuse to have a zoom plus a macro lens and even a portrait short telephoto lens in your collection!

Fixes and Tricks to Improve Close Focusing Distances

Recall that moving a lens forward, as on a bellows, improves its close focusing abilities. You can therefore mount a lens in an extension tube or a bellows and get much improved closer focusing and even macro photography uses out of it.

One trick frequently employed by portrait photographers is to use a thin extension tube to improve the close focusing distance of portrait lenses. For example, I use a thin 16mm extension tube with my 150mm Kowa 6 (6x6cm SLR) leaf shutter portrait lens (equal to roughly 100mm on 35mm SLR). Without the short extension tube, the lens doesn't focus close enough for tight head shot portraits. Many Hasselblad owners use a similar short extension tube with their Zeiss 150mm telephoto optics to get more close focusing range too. Of course, with the extension tube in place, the lens no longer focuses to infinity. But now you know why many relatively thin extension tubes are sold, even though you might think only thicker extension tubes would be of much use for macrophotography.

Another major user of short extension tubes are those long telephoto mirror lens users among us. Usually, a 500mm mirror lens has a rather long close focusing distance, perhaps from 30 to even 60 feet away. On a 500mm, that is still pretty close in terms of typical subjects. But if you want to do real closeups of small critters and birds, you need something to enable closer focusing. The solution again is often a simple thin extension tube to permit focusing closer with these long telephoto lenses.

Closeup diopter lenses are another trick for improving the close focusing distance of your lenses. One of my 100mm macro lenses only focuses to 1:2 (1/2 lifesize on film) directly. To get 1:1 (lifesize on film), I have to use an accessory matched closeup diopter lens with it. The nice part about closeup lenses is they don't change your exposure settings, as bellows and extension tubes behind the lens do (but TTL metering compensates). The image remains bright in the viewscreen too.

Fractional Diopter Lenses

The answer is fractional diopter lenses in strengths of from +1/4, +1/3, and +1/2 diopters. These closeup lenses effectively set infinity at 4 meters, 3 meters, and 2 meters respectively. These fractional diopter closeup lenses screw into the front of the lens, just like any filter. Their filter factor is 1, so they don't require any exposure offsets (like a clear UV filter).

When used with most short telephoto lenses, these fractional diopter lenses enable the user to get closer and still focus on the subject (which is larger as it is closer). The gain seems pretty modest, as a given short telephoto might go from 12 feet minimum focusing distance to 6 feet, or 6 feet to 3 feet in another example. But the size of the image on film will be potentially larger, overcoming and extending the limited close focusing distance of many lenses. One feature I like is that you can use the same closeup diopter lens on many different lenses and even different brands of cameras (unlike extension tubes), and even on smaller filter ring size lenses with the right step-up rings. Unfortunately, few folks know about fractional diopter lenses, so they are more rarely used and harder to find on the used market.

Conclusions

Related Postings:

[Ed. note: Thanks to Q.G. de Bakker for this correction (made above) - 
good news for me, since it helps reduce my "lens lust" to add a 180mm 
to my Hasselblad lens holdings ;-) ;-) Thanks!]
Date: Wed, 29 Aug 2001 
From: "Q.G. de Bakker" <[email protected]>
To: [email protected]

Hi,

I've just been reading your "Close Focussing Lens Issues"-page
(http://people.smu.edu/rmonagha/mf/close.html) and found the following piece
of text:

"Many Hasselblad owners use a similar short extension tube with their Zeiss
150mm telephoto optics, or opt for the 180mm Zeiss short telephoto which can
produce tighter head shots without an extension tube."

This actually is not true: the 150 mm lens, unaided, has a minimum field of 
view of 34.17 cm, while the 180 mm, unaided, has a minimum field of view of
34.48 cm. So the 180 mm does not produce tighter head shots without an
extension tube.

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