There is a very good article on the problem on www.photodo.com -
http://www.photodo.com/nav/artindex.html - Improved Sharpness
I remember somewhere reading that the critical "small enough hole" was 4mm
or less, depending on some other characteristics of the lens.
AndrewF
>Jamie,
>Tom's got it exactly. In camera lenses it's referred to as "diffraction
>limiting" and is a degradation of lens resolution created by the
>interference pattern from it.
>
>As an aperture is opened wider, the ray paths from a point in space to a
>point on film increase. The additional ray paths travel nearer the edges
>of the lens (versus nearer its axis). Wide open performance for most
>lenses usually isn't the sharpest because all of these ray paths do not
>meet exactly. As you stop the lens down, the number of ray paths decrease
>to those closer to the lens axis. Stop down to a small enough hole and you
>hit diffraction limiting in which the interference patterns causes enough
>ray paths not to meet exactly any more that it becomes detectable. That's
>why best possible resolving power for practical lenses is usually found at
>about f/8 or f/11; fewer ray paths closest to lens axis that meet more
>exactly before aperture diffraction kicks in enough to detectably spread
>them out.
>
>-- John
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