Diffraction and How it Will Affect Sharpness @ f/45

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The fact that the 300mm ED does not have an f/45 stop has led me to investigate how much deterioration will be caused by using that stop on the other lenses that do have it. The 300,400 and 600 Takumars were my concern. It seems that diffraction is more dependant on focal ratio than aperture size. In other words, the diffraction of the 200 Pentax at f/32 will be similar to that of the 600mm at f/32, even though the 600 has a much larger aperture diameter. There is a diffraction formula for determining the maximum resolution for lenses at various focal ratios. Optical aberrations don't need to be figured in because at that f ratio, they are negligible. It is 1390/f ratio. So, if we are concerned about using the old version 300mm at f/45, we can figure out that the lp/mm will be 31. Not really a bad figure and easily sharp enough for most publications.(That is if you don't have any shutter vib softening the image). Should the 300 ED have had the f/45 stop? I think so.

-- Steve Rasmussen (srasmuss@flash.net), April 27, 2001

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