Originally Posted by elekm
Top quality enlarging lenses often are designed to be diffraction limited at or near their largest aperture for the wavelength of light they are designed for. As a consequence, their optimal aperture is usually close to their largest aperture.
See some of the details and equations here:
Ordinarily light travels in straight lines through uniform air, however it begins to disperse or "diffract" when squeezed through a small hole (such as your camera's aperture). The resultant image softening effect is normally negligible, but increases as the aperture size decreases.
Photographers pursuing better sharpness use smaller apertures to achieve a greater depth of field, at some aperture the softening effects of diffraction offset any gain in sharpness due to better depth of field. When this occurs your camera optics (and enlarger optics) are said to have become diffraction limited. Knowing this limit can help you to avoid any subsequent softening, and the unnecessarily long exposure time or high ISO speed required for such a small aperture.
This should not lead you to think that "larger apertures are better," even though very small apertures create a soft image. Most lenses are also quite soft when used wide open (at the largest aperture available), and so there is an optimal aperture in between the largest and smallest settings-- usually located at or near the diffraction limit, depending on the lens. Alternatively, the optimum sharpness may even be below the diffraction limit for some lenses. The diffraction limit calculations only show when diffraction becomes significant, not necessarily the location of optimum sharpness (although both often coincide).
Everything is analog - even digital :D
You might want to read 'Post Exposure' by Ctein on this issue.
I have tested quite a few enlarging lenses with the same test negative. My tests verify Ctein's statements in his book. Here is my experience:
1. Stay away from 4-element designs, go for the 6-element lenses.
2. Name-brand lenses bare less risk but offer no guaranty. Well kept off-brands lenses often outperform ill-treated name-brand lenses.
3. Try before you buy. Enlarging lenses have large variations. Two lenses of the same design and manufacturer can vary more between each other then two lenses from different manufacturers.
4. Compare lenses. Just making a print with one and finding it to be OK doesn't tell the story. Only a side-by-side comparison allows you to enjoy a quality lens and spot the 'dog'.
5. Make sure the lens has a click-stop for every f/stop. That is very useful. Also, illuminated f/stops are a big help in the darkroom.
I understand the optical theory you're referring to, but this is not my experience at all.
Howard Bond told me a while back his lenses performed better wide open than stopped down. After reading Ctein's book, where he also states that enlarging lenses often perform best at wide open, I did some tests. Anyone can repeat this test very easily.
1. Put a negative into your enlarger and project a typical enlargement size.
2. Use your grain focuser to focus and observe the grain at wide open.
3. Now stop down one stop at a time and watch the grain getting fuzzy!!!
Nevertheless, the light distribution across the baseboard will get more even while stopping down. Just my experience in conflict with optical theory.
I've done test prints with some of my lenses, printing at a variety of f-stops. My results match what Tom describes; the optimal setting is usually around midway on the f-stop scale. The differences get smaller with better lenses.
A lens being sharpest wide open is not a contradiction to optical theory. Some aberrations improve as one stops down and diffraction gets worse -- on most lenses the balance between these two effects will cause the optimimum aperture to be a few stops down. This is a general and qualitative argument that doesn't give an exact answer for a specific lens. But on a very high quality (or very slow) lens, aberrations could be so small that the optimum aperture could be wide open. To know for a specific lens, you either have to test it, or do detailed calculations from the design data.
My observations with several top-quality 6-element enlarging lenses show them not to be at their best sharpness wide-open. They might be in the center, but the corners improve but stopping down a bit. So if you use a grain magnifier to study your enlarging lens, don't just look at the center. And, as Ralph says, it is very likely that the illumination will become more even if you stop down some.
Skimming the section in Ctein's book, I don't find him saying that enlarging lenses often perform best wide-open (as stated above). His table of test results mostly lists the optimum aperture as f4 for f2.8 lenses, f5.6 for f4 lenses, and f7 to f8 for f5.6 lenses. I only see one lens in his table that is listed with an optimum aperture that is wide-open: the 105 mm f5.6N Apo-El-Nikkor used for 135 format. This is a special case: an extremely expensive lens, and a longer than normal focal length. Plus, he comments that other lenses for the format, which have optimum apertures at f4, are sharper.
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FWIW, I did a printing session today and performed some more extensive tests on some of my 50mm lenses than I've done before, focusing on performance at a variety of apertures. I tested each lens at four apertures (wide open [f/3.5 or f/2.8], f/5.6, f/8, and minimum aperture [f/11 or f/16]) in both the center and the periphery. I tested a 6-element Nikon EL-Nikkor f/2.8, a 5-element Vega-11U f/2.8, a 4-element Durst Neotaron f/2.8, and a 4-element Industar-96U f/3.5. I used a negative I shot for this purpose last year; it's got text samples at various sizes in both the center and the periphery, shot on Ilford Pan F+ film, which makes it easy to judge sharpness. I found that the optimum aperture for the center exposure was f/5.6 or f/8 for all of the lenses I tested (most leaned towards f/5.6, but some were virtually indistinguishable between their f/5.6 and f/8 exposures). Note that I didn't test the f/4 aperture that MichaelBriggs mentioned. In the periphery, it was another story; most got clearer up to the minimum aperture. (Of course, the true optimum could be somewhere in-between f/8 and the minimum.) The Nikon's f/8 and f/16 results were indistinguishable, though, and the difference between f/8 and f/11 was pretty small for the Vega-11U. The periphery was also where the differences were most pronounced, both in terms of differences over the aperture range and lens-to-lens differences.
Overall, for my lenses I think using f/8 is the best compromise between center and edge sharpness if making uncropped enlargements in which edge-to-edge sharpness is important. When cropping or when subjects in the center or periphery are more important, this might change.
I did this test with B&W film and paper; I have no idea how color might complicate things.
Well stated and researched. Do we agree that stopped-down lenses are not the way to go?
The books all say, that enlarger lenses are best three stops from the biggest f/. That would be 8 or 11 in most cases.
I did this test with B&W film and paper; I have no idea how color might complicate things.
These books also say that in colour printing, changing exposures meant changing apertures instead of printing times. The time was to remain constant to prevent colour shifts which may arise from using different exposure times. I don't know if this still valid since I found no colour shifting when changing times in RA-4 printing.
If you want to shoot 6x9, plug for a 105 Rodagon. They are affodable and common...and good. I have a 80 Rodagon, 105 and 150 and all are darned good from wide open (tho I normally stop down 1-2 stops unless I cannot). I printed a 20x24 print from a 5x4 neg at f5.6-f11 using the 150 and could not see any difference anywhere. I have not tested the other lenses in this regard but al produce suprt crisp prints at 1-2 stops down which should be in ther sweet spot and certainly not any worse than other apertures. Also, using an f4 lens at f5.6 or 8 means that if you have heavy burning to do you can open up a stop or two to shorten burn in times (where you might have needed a longer main exposure for your manipulations).
I have only had one dog; a 50 f4.5 minolta which is suposed to be a good lens (tiny silver job) but vignetted VERY badly at all apertures and simply would not cover 35mm.
Michael is correct. Ctein's books refers to most lenses being at their best stopped down by just one stop. My use of 'wide-open' was too liberal. Nevertheless, my point was that the usual '2-3 stops down' rule is not verified by tests. Most enlarging lenses are better when wider open than that. I still suggest the test with the grain focuser. It will reveal the best performance for individual lenses. And yes, the test should be done in the corners and the middle, but many grain focusers can't get into the corners. Then again, why stop down further when the center is getting fuzzy already?