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[OM] diffraction

Subject: [OM] diffraction
From: Joseph <joseph@xxxxxxxxxxx>
Date: Fri, 28 Jan 2000 04:53:03 -0800 (PST)
=======
Anyone care to comment about this? A long lens that 
would degrade after F4 by diffraction?
=======

it means that this lens is so sharp that it is diffraction
limited at f/4.  diffraction limits resolution and most
lenses get better corrected as they are stopped down
and diffraction gets worse as they are stopped down.
this means that as you stop down a lens, it gets
sharper until diffraction takes over.  the sharper
the lens, the wider the aperture where this
crossover takes place.  most lenses are not
sharp enough at f/4 to hit the diffraction-limit
at f/4, which corresponds to an aerial resolution
of 400-425 lp/mm.  At f/11, the diffraction-limit
corresponds to about an aerial resolution of about
150 lp/mm.  so it takes a really well corrected lens to
be diffraction-limited at f/4.

joseph


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