On Mon, Sep 19, 2011 at 11:11 PM, Larry Colen <[email protected]> wrote: > > On Sep 19, 2011, at 8:02 PM, David Parsons wrote: > >> On Mon, Sep 19, 2011 at 10:50 PM, Larry Colen <[email protected]> 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. > > > -- > Larry Colen [email protected] sent from i4est > > > > > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and follow > the directions. > -- David Parsons Photography http://www.davidparsonsphoto.com Aloha Photographer Photoblog http://alohaphotog.blogspot.com/ -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
