diffraction
in [Digital Cameras]
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From: Martin Brown on 22 Dec 2009 10:45 Paul Ciszek wrote: > In article <hgpafg$gdv$1(a)news.acm.uiuc.edu>, > Doug McDonald <mcdonald(a)scs.jllinois.edu> wrote: >> The diameter of a diffraction blur spot at f/x is about x/2 microns FWHM. > > I only know undergraduate physics type optics; is it possible to arrange > a photograph with an "ordinary" camera that shows diffraction ripples as > seen in physics textbooks? Or does diffraction in photography just > take the form of a generic limit on sharpness? Yes. Just make the aperture mask small enough that diffraction dominates the image quality. A pinhole in aluminium foil will do nicely. Obviously you need a still life time exposure - preferably something with a specular highlight or point source in it. Regards, Martin Brown
From: David Ruether on 22 Dec 2009 11:06 "egbert_no_bacon" <egbert_no_bacon(a)hotmail.co.uk> wrote in message news:59ce6c6d-8b34-48f3-aaea-04e69a04302a(a)v30g2000yqm.googlegroups.com... > what is diffraction, and do you have an example or know where an image > of such is posted for me to see please If a lens were optically perfect (no manufacturing or design flaws (which is impossible, although some process lenses used for printing tiny circuits approach this), stopping down the aperture from wide open would progressively soften the image due to diffraction. Since lenses are compromises and stopping them down can help overcome some image flaws seen mostly at their wide stops, there are two conditions that therefore intersect - most lenses improve in performance as they are stopped down (and their flaws are progressively overcome) until the diffraction limit for a given stop is reached (after which diffraction progressively worsens the image quality with greater stopping down). The stop at which this occurs depends on the quality of the lens at its wider stops (stopping a very high quality lens below maybe f4 would begin to show diffraction softening with further stopping down, but putting an aperture in front of a simple magnifying glass may never show serious diffraction effects since orther image faults swamp diffraction). As for what diffraction is, it is the tendency of light to be bent as it passes an edge. The ideal purpose of a lens is to focus entering light rays coming from points in the subject to corresponding points on the sensor. If the aperture is wide, most of the area of the lens passing light has proportionally little diffraction effect on the light passed. With stopping down, a far higher proportion of the light passing through the lens will be affected by the diaphragm edges of the lens, causing the image points to become larger, resulting in a less accurate rendering of subject points on the sensor (and image resolution loss). For samples shot with a range of f-stops that show the difference diffraction changing with aperture can make in an image, go here -- http://www.donferrario.com/ruether/diffraction.htm --DR
From: Chris Malcolm on 22 Dec 2009 11:24 David Ruether <d_ruether(a)thotmail.com> wrote: > "egbert_no_bacon" <egbert_no_bacon(a)hotmail.co.uk> wrote in message > news:59ce6c6d-8b34-48f3-aaea-04e69a04302a(a)v30g2000yqm.googlegroups.com... >> what is diffraction, and do you have an example or know where an image >> of such is posted for me to see please > If a lens were optically perfect (no manufacturing or design flaws (which > is impossible, although some process lenses used for printing tiny circuits > approach this), stopping down the aperture from wide open would > progressively soften the image due to diffraction. Since lenses are > compromises and stopping them down can help overcome some image > flaws seen mostly at their wide stops, there are two conditions that > therefore intersect - most lenses improve in performance as they are > stopped down (and their flaws are progressively overcome) until the > diffraction limit for a given stop is reached (after which diffraction > progressively worsens the image quality with greater stopping down). > The stop at which this occurs depends on the quality of the lens at its > wider stops (stopping a very high quality lens below maybe f4 would > begin to show diffraction softening with further stopping down, but > putting an aperture in front of a simple magnifying glass may never > show serious diffraction effects since orther image faults swamp > diffraction). > As for what diffraction is, it is the tendency of light to be bent as it > passes an edge. The ideal purpose of a lens is to focus entering light > rays coming from points in the subject to corresponding points on > the sensor. If the aperture is wide, most of the area of the lens passing > light has proportionally little diffraction effect on the light passed. With > stopping down, a far higher proportion of the light passing through the > lens will be affected by the diaphragm edges of the lens, causing > the image points to become larger, resulting in a less accurate rendering > of subject points on the sensor (and image resolution loss). > For samples shot with a range of f-stops that show the difference > diffraction changing with aperture can make in an image, go here -- > http://www.donferrario.com/ruether/diffraction.htm And of course if you deliberately want to provoke more diffraction at wide stops normally free of diffraction effects, change the shape of the aperture restriction to one with a high edge to area ratio. -- Chris Malcolm
From: Wolfgang Weisselberg on 22 Dec 2009 14:08 Ofnuts <o.f.n.u.t.s(a)la.poste.net> wrote: > On 22/12/2009 13:02, Better Info wrote: >> If the glass isn't diffraction limited at its largest aperture, then that >> means it's not diffraction limited at ANY aperture. > For some value of "largest". Usually for false values, if the slime talks. > Are we taking about the "f-number" or about > the actual aperture dimensions? Neither, Worst (F)lies talks about the inherent superiority of P&S cameras. -Wolfgang
From: Wolfgang Weisselberg on 22 Dec 2009 14:10
egbert_no_bacon <egbert_no_bacon(a)hotmail.co.uk> wrote: > is f8 ok on all lenses No. P&S cameras will suffer. > can diffraction be seen at f8, ever Yes, with tiny (inherently more noisy) pixels, like P&S cameras use. -Wolfgang |