dmmartindale@watcgl.UUCP (Dave Martindale) (06/07/84)
The explanation for lenses being at their sharpest somewhere near the middle of their aperture range is this: Various aberrations produced by the non-ideal bending of light rays in the lens elements are reduced as the lens is stopped down and the more greatly-curved parts of the lens elements are no longer used. Thus things like chromatic aberration, coma, etc decrease continuously as the aperture decreases. However, the light rays are also distorted by diffraction as they pass the blades of the diaphragm. As the aperture decreases, the rays affected by diffraction form an increasing fraction of the total light reaching the film, causing a decrease in sharpness. So there is an optimum aperture for sharpness. Above that, lens aberrations reduce sharpness, and below that diffraction is responsible for reduced sharpness. If you want to see an extreme example of what diffraction can do, try shooting (or even looking) through a window screen.
annej@hammer.UUCP (Anne Jacko) (06/08/84)
I was also taught (in a photography program in college) that there is an optimum aperature for sharpness, and it is generally not the smallest aperature. There is some "formula" for telling what it is and I don't remember it without looking at my old notes. I just remember that for my 35mm camera, 55mm lens, it was f11. I also remember that for a 50mm enlarging lens for 35mm negatives the sharpest aperature was f5.6; again I don't remember how to figure that. -- A. Jacko, Tektronix
marcus@pyuxt.UUCP (M. G. Hand) (06/09/84)
<one for you and the rest for me... > Optimum sharpness aperture depends entirely on the design of the lens. Since with most lenses the designers strive towards a fast lens (ie one that has a large maximum aperture wrt its focal length, in this context), and since most photographs are taken somewhere in the middle of the f-stop range, the designers compromise somewhere in the middle, although it is *easier* to make a lens whose maximum sharpness falls at the small aperture end of the scale because you don't have to worry so much about spherical aberation, edge effects, internal reflections and all that bad stuff..... marcus hand (pyuxt!marcus)
wjhe@hlexa.UUCP (Bill Hery) (06/12/84)
Even if optimum sharpness could be attained at maximum aperture (as a previous poster claimed was theoretically true), practicality demands a partially stopped down lens in most cases. The shallow depth of field at say, f/1.4 or f/2.0, combined with the small focus ing screen will keep most of us mere mortals from getting that optimum sharpness. Bill Hery ATT Bell Labs, Short Hills
kar@ritcv.UUCP (Kenneth A. Reek) (06/12/84)
Try this if you have a grain focuser in your darkroom (aerial image variety
please, not the ones that project onto a ground glass or other screen).
Look at the grain while you adjust the aperature of your enlarging lens and
you will see that at very small aperatures the grains get fuzzy. Large
enlargement and unbearably grainy film like Tri-X help this to show up
better. By the way, I've seen this through my EL-Nikkor 50mm lens whose
optimum aperature is around f5.6.
--
Ken Reek, Rochester Institute of Technology
{allegra,seismo}!rochester!ritcv!kar