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Frank Keller wrote: "After looking through one of the latest photography magazines I have a question. Several dealers had a 70-210 2.8 for sale. My question is this, this lens was followed by the letters APO. What does this mean?" APO stands for "apochromatic". This lens contains glass which corrects the phenomena of chromatic aberration. This is the tendency of light to slightly diffract in longer lenses. This causes the various wavelengths of visible light to focus at slightly different distances, depending on wavelength. In other words, with a very long non-APO lens, red and blue light will focus at slightly different depths with respect to the film plane behind the last piece of glass. If you shoot wide open with a non-APO long lens the red and blue light will not be simultaneously in focus at the film plane. You could get one in focus and the other would be focused somewhat in front of or behind the film plane. This makes the pictures appear soft. Stopping down to f8 or more helps with a non-APO lens, but buying an APO lens is better. I used to own a Sigma 400mm non-APO lens and found it suffered from this phenomena. I sold it. I am not familiar with your particular camera, but it would not surprise me if it did not accurately autofocus with a non-APO lens. A typical autofocus mechanism works by comparing contrast at the film plane, in front of and behind the film plane. The servo motor attempts to move the sharpest contrast to the film plane. A sharply focused image contains the most contrast. Since the non-APO lens puts the focused light of different wavelengths in different planes, the auto focus will probably not work accurately. I could not autofocus with the Sigma 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 =======================================================
