An utterly superb tutorial* on diffraction limits as applied to camera and other lenses, examples, calculators, hover to see demonstrators and more. http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm Extract: As two examples, the Canon EOS 20D begins to show diffraction at around f/11, whereas the Canon PowerShot G6 (compact camera) begins to show its effects at only about f/4.0-5.6. On the other hand, the Canon G6 does not require apertures as small as the 20D in order to achieve the same depth of field (for a given angle of view) due to its much smaller total sensor size (more on this later). More extracts from this page at end of this email. Note that "begins to show" is far from the limit of its useful range. See web page. * Hint: "tutorial is a mis-spelling of "primer". ______ 1. Admin hat on: > > ... BTW, regarding the word "existence", I wonder, does ... It is explicitly against both the letter and spirit of the list rules to mount explicit, implicit or even vaguely adumbrated "argumentum ad hominem" attacks on list members. Please refrain from doing so on list. By all means criticize list members about their spiiling mistooks and the size of their mother's army boots offlist in private email. Hint: Don't do this with someone with too many web smarts or vast technical capability at any geographic remove, no matter how great. ********* Admin hat off and locked away. *********** 2 >> www.questarcorporation.com/QuestarPDF/QM_100_30003.pdf >>> Operating range of f/no: 6.0 to 3.5 (less than 14 or 22) VERY nice toy. >> Those figures are the maximum possible aperture (= minimum >> aperture number) at varous distances. Smaller apertures can >> usually be used in practice at any given distance - just not >> larger ones. > Yes, but that was not a telescope, that was a microscope; there was no > need to catch as much light as possible (at least for that price). The > very reason for the existence of the big apertures must be - better > resolution, I believe. My point (possibly wrong) was that "diffraction > could be the major issue at f/14", not to say at f/22. It is actually a telescopic microscope - ie it is a microscope whose forte is much greater distances to object than is usually the case. Questar usually deal with objects at light-years remove but have lowered their range in this case. A system with the largest possible aperture as a starting point is, all else being equal (which it never is), liable to be superior to one with smaller maximum aperture. A system which does not have constant aperture across the range - eg f 1:2.8 all the way, almost always indicates that a compromise has been used between performance and cost. The "Gold Standard" is constant aperture across zoom range and this is what you get in most top zoom lenses. If you don't it's usually because they are 'pushing the envelope" in some other area. [[I own NO 35mm zoom lenses with constant aperture. The closest I have is a superb Minolta 17-35mm f2.8-f3.5. This is such a limited range lens that it is about the only one that Sony decided not to build when they took over Minolta's camera division. Probably the best lens I own. ]] So. I stand by what I said technically, so far. FWIW. An utterly superb tutorial* on diffraction limits as applied to camera and other lenses, examples, calculators, hover to see demonstrators and more. http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm Hint: "tutorial is a mis-spelling of "primer". ______ >From above tutorial / primer: Recall that a digital sensor utilizing a bayer array only captures one primary color at each pixel location, and then interpolates these colors to produce the final full color image. As a result of the sensor's anti-aliasing filter (and the Rayleigh criterion above), the airy disk can have a diameter approaching about 2 pixels before diffraction begins to have a visual impact (assuming an otherwise perfect lens, when viewed at 100% onscreen). As two examples, the Canon EOS 20D begins to show diffraction at around f/11, whereas the Canon PowerShot G6 (compact camera) begins to show its effects at only about f/4.0-5.6. On the other hand, the Canon G6 does not require apertures as small as the 20D in order to achieve the same depth of field (for a given angle of view) due to its much smaller total sensor size (more on this later). Since the size of the airy disk also depends on the wavelength of light, each of the three primary colors will reach its diffraction limit at a different aperture. The calculation above assumes light in the middle of the visible spectrum (~510 nm). Typical digital SLR cameras can capture light with a wavelength of anywhere from 450 to 680 nm, so at best the airy disk would have a diameter of 80% the size shown above (for pure blue light). Another complication is that bayer arrays allocate twice the fraction of pixels to green as red or blue light. This means that as the diffraction limit is approached, the first signs will be a loss of resolution in green and in pixel-level luminance. Blue light requires the smallest apertures (largest f-stop number) in order to reduce its resolution due to diffraction. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist