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 Originally Posted by David A. Goldfarb
Dan, what you are saying is true in the macro range, but just for accuracy's sake, it's not true as the focal point approaches infinity. At a magnification of around 1:1 or even 1:5 and pretty much at 1:10 all lenses of all focal lengths have effectively (i.e., calculated out to four significant figures) the same DOF on the same format and at the same aperture. So why use a longer lens for macro if possible?--more working distance for lighting. Why use a shorter lens?--not enough bellows for a longer lens at the desired magnification.
However, if the magnification ratio is small, like 1:100 or 1:2000, it is true that a wider lens has more DOF than a longer lens at the same aperture and for the same magnification.
David, what you say is true and well known. That said, its practical significance is small.
For those of you who aren't clear on what we're discussing, the question is whether shooting a distant scene with a long lens and enlarging less or shooting the same distant scene with a short lens and enlarging more will give more DOF in the final print, with the final prints showing the same scene. As David wrote, using a short lens and enlarging more will indeed give more DOF in the final print, but there's a snag. The snag has to do with how much enlargement the negative will allow before acceptable image quality in the print is lost.
In practice, shooting with a short lens and enlarging a lot is not a kind of free lunch. If it were, we'd all shoot the shortest lens that covers with the movements we need and then enlarge as needed.
Choice of focal length for closeup work is driven, in my experience, by the need for working distance. There's a conflict between working distance and the extension needed to get it, and longer lenses tend to be larger and heavier too, so we rarely use really long lenses for closeup work. For magnifications above 1:1, it seems that lenses much longer than 100 mm are rarely used.
Also in my experience, different subjects can tolerate different working distances. For example, I sometimes shoot small parts of little fish. Dead fish, in alcohol to control specular reflections. My shortest lens, a 17/4 Tominon would be ideal for this, but I'm reluctant to risk wetting it, so I use a somewhat longer lens in that situation. For another example, those of us who shoot insects and such out-of-doors with SLRs try to use the longest lens we can manage. Skittish subjects that can get away don't like having a lens in the face. Same goes for fish in aquaria. Side point, an SLR is a much better tool for shooting mobile subjects closeup than is, say, a view camera.
Finally, the kinds of portable flash rigs I like to cobble up are problematic with my flashes (not powerful enough) and long lenses. Basically, when shooting closeup I compromise and generally can't quite win.
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quote: "For those of you who aren't clear on what we're discussing, the question is whether shooting a distant scene with a long lens and enlarging less or shooting the same distant scene with a short lens and enlarging more will give more DOF in the final print, with the final prints showing the same scene. As David wrote, using a short lens and enlarging more will indeed give more DOF in the final print, but there's a snag. The snag has to do with how much enlargement the negative will allow before acceptable image quality in the print is lost."
Perhaps I am not the only one who is confusing matters. (I believe that oranges and pineapples were mentioned.) I don't find any mention made of enlarging or printing as I read this post until you interjected these factors...no mention made of degree of enlargement, depth of field in a print or any of the other issues that you seem to be interjecting at this point. Since degree of magnification by definition would include any and all departures from the actual image size as it exists in reality and represented on film, the fact still remains that at infinity focus a shorter focal length lens will give more depth of field then a longer focal length lens. This increased depth of field exists on the negative and not solely on the print as I understand you to be inferring. I disagree with your position that this is an insignificant matter especially when I have worked to gain depth of field from 6 ft to infinity with a 450 mm lens on 8X10 (yes that is negative size). This awareness is especially true when I compare the depth of field constraints of this longer lens to a shorter 159 mm lens on 8X10 and contact printed (you will notice that i make not mention to enlarging here).
"In practice, shooting with a short lens and enlarging a lot is not a kind of free lunch. If it were, we'd all shoot the shortest lens that covers with the movements we need and then enlarge as needed."
Once again you seem to be thinking from within the box of thought relating to enlarging. For a number of us, we do contact print large negatives. We use lenses that will allow us to maximize depth of field consistant with a composition of our choosing. Most of us don't even think of cropping a portion of the negative because enlarging doesn't enter our frame of reference. We work to move the camera to the best possible position in consideration of the object or scene to be photographed, the presentation that we wish to make and yes the focal length of the lens.
"Choice of focal length for closeup work is driven, in my experience, by the need for working distance. There's a conflict between working distance and the extension needed to get it, and longer lenses tend to be larger and heavier too, so we rarely use really long lenses for closeup work. For magnifications above 1:1, it seems that lenses much longer than 100 mm are rarely used."
I am wondering what format you are describing when you make the statement above. Are you talking about 35 mm or medium format? I wonder about this since you have a penchant to approach this from the standpoint of enlarging. While I have no basis to argue with your personal experience, I would go on to say that working distance is not the only consideration....nor is it the primary consideration in many cases. In the area of product photography, for instance, a 90 mm lens on 4X5 will give a totally different product presentation then a 210 mm lens. Looming distortion and foreshortening will be totally different between these two lenses. Sometimes a 90 mm lens will not allow adequate lighting controls to be employed. In those cases there are certainly images photographed with lenses that exceed 100 mm...certainly true with 4X5 and larger.
In consideration of your statement about maximum focal length rarely exceeding 100 mm, I will say that on my 8X10 with 28 inches of bellows extension a 159 mm lens will allow me magnification exceeding 2.0 On my 4X5 with 20 inches of bellows extension a 150 mm lens will allow magnification in excess of 1.5.
I can certainly understand your strongly stated position if you are drawing from the limited perspective of smaller formats...however that is not true of all of photography.
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Pete, you may want to download and print the Quickdisc from: http://www.salzgeber.at/disc/ (if you have not done so already). Not only will this give you the correct adjustment for bellows extension but you can use it as Dave suggests and focus on the text on it...
Cheers, Bob.
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Donald, thanks very much for disagreeing so strongly with me. You forced me to reconsider what I thought I believed.
I've done the arithmetic, using magic formulas lifted from http://www.dofmaster.com/dofjs.html . First off, my Excel spreadsheet doesn't give exactly the same answers their java machine does. I've checked, can't find what I'm doing wrong. In any case, the answers I get are close to theirs, but not exactly the same. And they're very interesting.
It turns out that you're right when the shorter lens' focused distance is around half its hyperfocal distance (depends on focal length, relative aperture, desired circle of confusion) and the longer lens' focused distance is well inside its hyperfocal distance, both lenses set up to capture the same scene. In this case the the shorter lens will indeed give more DOF than the longer one.
It turns out that I'm right when both lenses are focused at much less than half their hyperfocal distances and capture the same scene; then they give very nearly the same DOF. Yes, the shorter lens gives more, but not a lot more. For example, at f/22, CoC = 0.03 mm, a 300 mm lens focused at 30m gives DOF of 13.7m and a 600 mm lens focused at 60m gives DOF of 13.2m. I'm not sure 50 cm is a big deal in that situation.
None of this has to do with format. Its all about focal length.
As for use of long lenses for closeup work there are process lenses made for use at 1:1 at least as long as 2.5m. They can be used as high as 3:1. My tiny little cameras can't handle one of those monsters, and neither can yours.
The longest macro lenses sold for general photographic use are arond 200 mm, and most, if not all, have maximum recommended magnification of 1:1. The major exceptions are ~ 100 mm Luminar, Photar, Neupolar, all of which will go to around 8:1 with good image quality. There's close, and then there's really close. My tiny little cameras won't reach 8:1 with my 100 Neupolar, and neither will yours. There are limits, alas.
Cheers,
Dan
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