OK, but how/why are you concluding that "diffraction intrusion" (your term, not mine) is the culprit at only F6.3-F7.1 when diffraction limiting effects would be on the order of 200-240 lp/mm at that aperture? I dont think what your seeing or the results you got could have had anything to do with diffraction effects if you shot at f6.3 to f7.1 and on a pentax DSLR because the sensors are nowhere near the diffraction effects resolution at that f-stop and for you to see a difference, the "intrusion" problem would have to be huge to even see it relative to the diffraction effects at that f-stop.
Regarding the "2mm" rule, it makes no sense. when you use a 1000mm lens you would be shooting at F500! Or even a 100mm lens you would be at F50! Thats not right. If your talking only about the 14mm lens, then its F7 but why use a rule that only works for one lens? The general "rule" I have always heard of and found to be pretty good generally is to set the lens 2-3 stops down from wide open for best resolution regardless of focal length or absolute aperture which is what your "2mm" rule is. I find the 3 stops is typically better for very fast or wide angles and 2 stops is better for slow or normal to telephoto lenses. This all ignores DOF issues of course... jco -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Godfrey DiGiorgi Sent: Friday, December 15, 2006 12:15 AM To: Pentax-Discuss Mail List Subject: Re: K10D diffraction limiting On Dec 14, 2006, at 8:59 PM, Digital Image Studio wrote: > On 15/12/06, J. C. O'Connell <[EMAIL PROTECTED]> wrote: >> The diffraction limits of optical lenses are >> independent of focal length and absolute aperture. Diffraction limits >> are inversely proportional to f-ratio (f-stop) and the approx formula >> is 1500/f-ratio in line pairs /mm. This means at f6.3 lens would have >> to first resolve ~240 lp/mm to be considered diffraction >> limited and that would be quite an impressive lens >> indeed. Most lenses dont reach true diffraction limited >> resolution performance until f11 or smaller. e.g. >> they are WORSE than the diffraction limit as f11 or wider. >> I dont think you are going to find any 14mm lenses >> for 35mm FF or even APS that are truly reaching diffraction >> limited performance across entire format ( as good as phyics will >> allow >> ) at >> an f-stop as wide as F6.3. Aint going to happen on such a wide >> angle lens, even on APS where its not as wide as 35mm. > > Sometimes you make a great deal of sense John. This be one of those > times. ;-) I have test photographs the demonstrate the onset of diffraction at f/ 6.3 to f/7.1 with the DA14mm lens. This is not diffraction limited, it's the point at which diffraction begins to intrude faster than the lens' resolution improves. "Diffraction Limited" is a different thing ... A good enough explanation of the difference: "Diffraction is the ultimate limit to optical performance which cannot be overcome. Even if all aberrations were perfectly corrected there is still the diffraction which keeps the optics from reaching higher resolution. When using the term 'diffraction limited' for a scientific instrument or a lens, it means: this optic is so good that it is limited by diffraction only." (Kornelius Fleischer, Photo.net, 9/3/99) So aside from the nomenclature challenge, I empirically use the notion of approximately 2mm as my minimum lens opening for best lens performance at normal subject distances. It has never done me wrong. Sorry for the confusion. Godfrey -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
