My apologies if this appears twice, but I sent it yeaterday and messages from 12/21 are already appearing in the digest....so I will give it another shot as it has not yet appeared
I was reading a book which had a section on macro work, and it mentioned that because of light loss on macro lenses without internal focusing that one should not stop down smaller than f 11. The reasoning was that with such light loss when the lens changes length that the true f stop was not given and that f 11, when you are at a 1:1 image size, is the equivalent of approximately f 22 in 'true' aperture and any less than f 11 may therefore start to degrade image sharpness because of diffraction. I have one of the older Canon 100mm f 2.8 macros, the one without USM, that changes length when focusing. I had always been stopping down to f 22 when I wanted to attain maximal DOF and the camera was tripod mounted so longer exposures were not of concern in the absence of wind. But now I find myself wondering if instead I am "shooting myself in the foot" by actually using an exceedingly small effective aperture and causing degradation of sharpness and image quality by diffraction. With a lens like this is it, in fact, the case that f 11 at 1:1 size is equivalent to f 22 in terms of diffraction? If so, should most macro work using this lens be done at f 11 and just consider that DOF is maximized at this level and any smaller yields little DOF gain but other problems like diffraction? Howard * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
