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
At 11:34 10/13/01, Tom Scales wrote:
If a word: diffraction.
Here's a quote I found in a quick search:
"When a beam of parallel light passes through a circular aperture it
spreads out a little, a phenomenon known as diffraction. The smaller
the aperture, the more the spreading. "
with more detail (most of which I don't understand) at:
http://www.faqs.org/faqs/rec-photo/lenses/tutorial/
Tom
>
> > ... Forgetting for a minute that lenses aren't critically sharp at
> f/22, ...<
>
> Jim (or anyone else):
>
> Can you explain to us newbies why this is so?
>
> Jamie
>
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