Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site watcgl.UUCP 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.