Archived posting to the Leica Users Group, 2010/11/28
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]It seems like diffraction is related to the absolute size of the aperture rather than the F-stop which is a ratio. A long focal length can be stopped down to F45 or F64 on a large format sensor. On a pocket digicam with a real short focal length they rarely go smaller than F8. I have a small video camera at work with 1/2 inch chips and there is a noticeable drop in sharpness past F8. I hadn't noticed this as much with the larger 2/3 inch vid cameras. Mike D On 11/28/10 1:58 AM, afirkin at afirkin.com wrote: > Its the potential to cause problems with the photographic image and esp. > now with digital sensors which interests me. Not that many of my lenes > will stop down below f16/ f22 and mostly I'll use f8 or wider, but with > the creek shots etc I was using f22 to increase exposure. Perhaps I'd be > better with some ND filters. > > Cheers > > Alastair > >> Diffraction occurs on the edge of the iris diaphragm. Because the diameter >> of the aperture opening is determined by 2??r but the area of the aperture >> is determined by ??r??, the greater the radius, the greater the >> area:diameter ratio. Thus, the smaller the radius of the aperture, the >> greater the impact of diffraction. The shorter the wavelength, the less >> the diffraction, but it isn't much of a factor until you get to >> 1,000-power on a microscope. >> >> >> On Nov 27, 2010, at 6:11 PM, afirkin at afirkin.com wrote: >> >>> Diffraction Limits: >>> >>> The diffraction limit is dependent on the wavelength of the light and >>> the >>> f-stop. It is calculated as the size of an Airy disk (Sir George >>> Airy). >>> This describes the size of a circle from a point of light as it passes >>> the edges of the aperture. Fundamentally, as you decrease the aperture >>> size (increase the f-stop) the size of the Airy disks increases. An >>> Airy >>> disk can also be called a blur circle at the image plane similar to the >>> CoC for DOF. The simplified calculation (estimation) is 1500 divided >>> by >>> the aperture. Thus, f/8 yields .005 mm and f/64 yields .042 mm. The >>> relationship between the size of the Airy disk and resolution is known >>> as >>> the Rayleigh criteria. Since it takes two of these points of light to >>> define resolution the respective resolutions are 100 lp/mm for f/8 and >>> 12 >>> lp/mm for f/64. Obviously, stopping down the aperture to increase >>> depth >>> of field can lead to increased diffraction that blurs the entire image. >>> >>> Most photographers would agree that 35mm cameras become diffraction >>> limited at f/22. At smaller aperture sizes diffraction trumps depth of >>> field and the overall image loses resolution. For a typical digital >>> sensor size the diffraction limit is a factor at f/16. Diffraction >>> limits are less significant with medium and large format cameras. This >>> is only because there is less need for enlargement therefore the CoC >>> values can be relaxed. >>> >>> >>> _______________________________________________ >>> Leica Users Group. >>> See http://leica-users.org/mailman/listinfo/lug for more information >> >> _______________________________________________ >> Leica Users Group. >> See http://leica-users.org/mailman/listinfo/lug for more information > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information >