======================================================= -> This is The 'SPORRS' Mailing List -> Info File: http://www.anet-stl.com/acphotog/sporrs/infosporrs.htm -> Note: Remember to include your name in each list post or reply. -> Please delete all unnecessary quoted text from the original message! =======================================================
RICK NEWTON writes: "I dangerously assume that you had the f5.6 version of the lens. f5.6 is typically the slowest aperture that any autofocus camera will work with, and does not always work well.. Autofocus cameras use a phase matching system to determine the focus, basically matching multiple images together to determine correct focus. I may be incorrect, but the APO feature of the lens is less likely a cause of autofocusing failure, than the slow lens." Answer: That is actually a very good possibility, but I think it was due to something in the non-APO glass. Here is why. I shoot the EOS system. Canon cameras automatically disable AF when they detect a lens with a maximum aperture smaller than f5.6. I have experienced that little joy, too. It just runs the lens all the way out to maximum focus and then all the way in to the minimum focus without locking on anything. No matter how optimumly textured the focusing object is, like a brick wall, the camera cannot and will not find a lock. It is actually not even trying. With my Sigma 400mm f5.6 non-APO lens the camera would quickly and repeatably find a focus lock. Unfortunately it was always about 10 percent behind what the desired target was. The difference in behavior of the system between a smaller than f5.6 aperture lens and the non-APO lens led me to believe that it was indeed the non-APO feature which caused this particular problem. Not knowing exactly what wavelength of light the contrast sensors in the autofocus mechanism were most sensitive to, I assumed, perhaps incorrectly, that the device was indeed correctly focused on whatever wavelength it was looking at. However, to my eyes, it was obviously not focused in the visible spectrum. The slides shot were obviously not focused either. It was not a case of gross overall softness as some elements in the image would be sharp, while the desired object was blurred. (My wife's all time favorite image of herself was accidently achieved this way. Her face is rendered somewhat soft, while the trees and leaves behind her are sharp. I always thought the image was no good because it is out of focus, but she loves it for the soft look to her face. Obviously I am not a portraiture specialist.) I found eventually that setting this lens to f8 or smaller and using manual focus was the best way to work with it. It just could not accurately focus with my system. If someone else has a better explanation, I would be interested. In general, I would caution anyone who uses an AF system to stick with the lenses made by the same manufacturer as their camera bodies, and don't buy a long non-APO lens. Mark Bailey Rockford, IL ======================================================= -> SPORRS: 'Serious Photographers Of Railroad Related Subjects' -> Web Site: http://www.anet-stl.com/acphotog/sporrs/ -> Message © 1998 SPORRS® - All Rights Reserved =======================================================
