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Path: utzoo!watmath!watcgl!dmmartindale
From: dmmartindale@watcgl.UUCP (Dave Martindale)
Newsgroups: net.rec.photo
Subject: lenses are sharpest at medium aperture
Message-ID: <2633@watcgl.UUCP>
Date: Wed, 6-Jun-84 17:24:17 EDT
Article-I.D.: watcgl.2633
Posted: Wed Jun  6 17:24:17 1984
Date-Received: Thu, 7-Jun-84 02:32:57 EDT
References: <697@decwrl.UUCP>, <287@tpvax.fluke.UUCP>
Organization: U of Waterloo, Ontario
Lines: 20

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.