What I remember about diffraction is that it's mostly related to the absolute diameter of the iris opening, so a 600mm lens at f:128 would have roughly the diffraction of a 60mm lens at f:12.8, so it should be usable.
What I remember about diffraction is that it's mostly related to the absolute diameter of the iris opening, so a 600mm lens at f:128 would have roughly the diffraction of a 60mm lens at f:12.8, so it should be usable.
Well, I know a thing or two about this lens that hasn't changed in substance over the past 16 years that this ancient thread has been around. I sometimes make 30X40 inch full gloss color prints from 8X10 film and the 600C lens. Yes, at that scale, there will be a modest discernible difference f/64 versus f/45 or f/32. The diffraction effect is real, but negligible in smaller print sizes. Contact prints are a no-brainer; you wouldn't be able to tell the difference in a contact print even if the aperture on this particular lens were capable of being stopped down to f/256. Unevenness of plane in a conventional film holder, or sag, is a far bigger real-world issue than modest distinctions in aperture.
AFAIK, the amount of (measurable, if not always visible) diffraction decreases at a given aperture as focal length increases, and does so in proportion to focal length. This also means that diffraction limitation values, at given aperture values, will likewise change with FL, making the generalizations about specific aperture values (like "be careful beyond f/32") moot if not also specified per focal length.
Do keep in mind that this is completely irrespective of the host of other variables (environmental, mechanical, chemical, etc. etc.) with which we in this community have or will become intimately involved...like it or not!
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