On Sep 19, 2011, at 8:30 PM, David Parsons wrote: > On Mon, Sep 19, 2011 at 11:11 PM, Larry Colen <l...@red4est.com> wrote: >> >> On Sep 19, 2011, at 8:02 PM, David Parsons wrote: >> >>> On Mon, Sep 19, 2011 at 10:50 PM, Larry Colen <l...@red4est.com> wrote: >>>>> >>>> Diffusion or diffraction? >>>> It was my understanding that diffraction happens at any opaque edge within >>>> a >>>> range that is determined by the wavelength. Therefore the larger the >>>> aperture the smaller the percentage of it that will be subject to the >>>> effects of diffraction. >>>> >>>> f/64 on a 320 mm lens would be an aperture of 5mm, which would be the same >>>> as f/10 on a 50mm lens or f/8 on a 40mm. >>>> >>>> I suppose that it's possible that the angle of incidence of the light beams >>>> hitting the aperture could affect the amount of diffraction, and therefore >>>> it's not a linear function that diffraction becomes a problem with >>>> apertures >>>> of 2mm or smaller in diameter. >>>> >>>> It's also been roughly 30 years since I took a course in either physics, or >>>> field equations, so I could be completely off base. >>> >>> Aperture values are ratios, not absolute values. f/64 on LF is going >>> to be a different size than f/64 on 35mm. >>> >>> On a 320mm lens, f/64 has an aperture diameter of 5mm. On a 50mm >>> lens, f/64 would have an aperture diameter of .78mm. (This doesn't >>> account for the fact that the aperture blades aren't necessarily at >>> the nodal point in a lens, and you are talking about a simple lens, >>> reality is a bit messier.) >>> >>> Your numbers are completely wonky. >> >> 320mm/64 = 5mm, 50mm/10 = 5mm, 40mm/8 = 5mm >> >> How is that wonky? > > Because you are comparing different apertures and calling them the > same. The physical opening may be the same, but you are dealing with > different focal lengths and formats, and that changes the > characteristics of the aperture. Diffraction limits depends on your > image/sensor size. Larger formats can stop down much farther before > diffraction becomes an issue.
My premise was that diffraction was an edge effect and that the ratio of edge to area was the critical factor. If that was the case, then it is the physical size of the aperture that is critical, not the f/stop, and therefore my math was correct. You didn't explain why it wasn't correct, just said that it wasn't. It is, however, explained at: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm Technical Note: Since the physical size of the lens aperture is larger for telephoto lenses (f/22 is a larger aperture at 200 mm than at 50 mm), why doesn't the size of the airy disk vary with focal length? This is because the distance to the focal plane also increases with focal length, and so the airy disk diverges more over this greater distance. As a result, the two effects of physical aperture size and focal length cancel out. Therefore the size of the airy disk only depends on the f-stop, which describes both focal length and aperture size. The term used to universally describe the lens opening is the "numerical aperture" (inverse of twice the f-stop). There is some variation between lenses though, but this is mostly due more to the different design and distance between the focal plane and "entrance pupil." The above page also explains diffraction limiting in similar terms as the calculation for Depth of Field, i.e. circle of confusion, based on sensor resolution, viewing distance etc. APS-C (1.5x) at 10.8 MP, f/11 has a pixel size of 5.9um, circle of confusion and airy disk of 14.7 um 35mm at 24.9 MP has pixel size of 5.9, coc and ad of 14.7um An 8x10 with 1420 MP, f/ll has a pixel size of 5.9um, coc and ad of 14.7um An 8x10 with a 42 MP sensor, f/64 has a pixel size of 34.1um, coc and ad of 85.2um. The short answer is that it's not the physical diameter of the aperture, nor the physical size of the sensor that matters, but the f/stop and the resolution of the sensor (or film). The reason that you can use a higher f/stop with large format has nothing to do with the size of the sensor, or the length of the lens, just the size of the individual pixels (or grains). -- Larry Colen l...@red4est.com sent from i4est -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
