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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