Thanks Leonard. This is very comprehensive. I may have to read it a few more times with camera at hand, so I can practice. This is the type of explanation I was looking for.
Your calculation also comes up with more realistic apertures. I just read in another thread you posted, that your calculation is based on a given viewing distance of the final print. So I'm assuming if I wanted a closer viewing distance I would have to alter the calculation.
That's the rub sometimes. Due to the nature of the scene, sometimes the choice is a very small aperture or no photo. I just decided that if diffraction really turns into a problem, I would not enlarge a photo that I like to the point that it becomes an issue. In practice, since most of my photos are not enlarged from 4x5 to more than 11 x 14 that it has not made a practical difference. In the meantime, I got some photos at f/45 that would not have worked as well with a larger aperture.Originally Posted by niubi
As i understand it at f/45 with say a 150 mm lens your aperture diameter works out to a bit over 3 mm. I find it hard to imagine diffraction would be an issue since this is a few thousand times greater than the wavelength of visible light.
For most apertures on our large format cameras I can't see how diffraction would be an issue. Am i wrong?
The advantage to getting the articles, or one of these books
Large Format Nature Photography by Jack Dykinga
User's Guide to the View Camera by Jim Stone
Using the View Camera that I wrote
is that there are illustrations and diagrams to go along with the written material. Some of us, and I am one of them, learn better visually, and the pictures will help a lot.
If you like the math fine but it is not necessary to use, or even understand it, to solve your problem.
steve simmons
www.viewcamera.com
Dan,
Diffraction is measured by the size of the Airy disc, or alternately in terms of effective lp/mm as measured on the film. These depend on the relative aperture or f-number, not simply on the physical dimensions of the aperture. Both the physical size of the aperture and the distance of the film from the lens enter into the calculation and it is the ratio of the second to the first that matters. When focused at infinity, this ratio is the f-number. When closer than infinity, this ratio is the f-number multiplied by one plus the magnification, which is called the effective f-number. But, except when one is in the close-up range, the effective f-number is so close to the f-number that it doesn't pay to worry about the difference.
The usual rule of thumb for photography is that the resolution limit imposed by diffraction at the film is 1500 divided by the (effecitve) f-number. For f/45, this is about 33 lp/mm. This is less than a good large format lens can deliver at its optimal aperture, so diffraction at f/45 will typically play a significant role. The way this enters in DOF considerations is that you won't get quite the desired resolution at the limits of the specified DOF region as you would think based just on defocus calculations. By stopping down further, you will in effect extend the diffraction-free DOF region, thus increasing the resolution because of defocus at the place you need it. The aim is to calculate just how far you have to stop down to counterbanace the effect of diffraction. To get this right requires sophisticated analysis based on MTF curves, But one can come close using certain rough rules of thumb, and that is what Hansma did. Perhaps someone who is a better physicist than I am can provide a more intutitive argument explaining this. But it doesn't really matter in the end whether the results of such a calculation square with our intuition.
The usual calculations are based on an 8 x 10 print viewed at about 10-12 inches, which is the normal distance most people would view such a print. Indeed, it is as close as most adults can get. If you produce a larger print, and get proportionately further away, the calculations for an 8 x 10 print still hold. But if you still view the larger print from 10-12 inches, as many people would do, in principle, you would have to take that into account. But some sample calculations show that this would lead quickly to unrealistic apertures. For example, suppose a coc of diameter 0.2 mm (corresponding to 5 lp/mm) is acceptable at the level of the print. If you enlarge 2 x for an 8 x 10 print, that would require 0.1 mm (corresponding to 10 lp/mm) in the film. But if you enlarged an additional 3 X for an 24 x 30 print, and still viewed that at 10-12 inches, you would need a coc at the level of the film of 0.033 mm (correponding to 30 lp/mm). To obtain that at the limits of the DOF region would require stopping down an additional 3 stops. If f/32 worked for an 8 x 10 print (or a larger print viewed further way), then you would need something like f/90 for the larger print, and that is ignoring diffraction. Clearly, this is impractical.
The upshot is that if people insist on viewing large prints from close-up they are going to see much reduced depth of field. Even in the exact plane of focus, where the resolution would be limited by lens aberrations and diffraction, there is a practical upper limit for how far you can enlarge without loss or quality in the print viewed close up. For example, if you stop down to f/32, just from diffraction, the resolution at the film level would be about 47 lp/mm. The optimum resolution ignoring diffraction might be 60 lp/mm. The combination would result in something between 26 and 36 lp/mm. If 5 lp/mm were acceptable to you when viewing the print, you would be limited to enlarging between something like 5 and 7 times, and if you did, you would end up with zero DOF. It gets even worse if you take into account the resolution of the film.
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