Here is some more information about my artificial light front focus issue... Much like what Roger has stated... The Sigma just may not be well corrected for this with the Canon AF system and would happen with every single Sigma 35... Not copy variation. But due to the different AF systems, it may or may not affect certain bodies I would think...
New in the Canon EOS 7D is a feature that tweaks the behavior of the AF system based on color information from the new color-sensitive exposure sensor, reducing AF problems often encountered when shooting under artificial illumination. Canon staff told us that back-focus errors can relatively common under artificial lighting, particularly in stadiums or other sporting venues. Canon didn\'t go into details of why this might be the case, or what specifically they did to correct it, but it\'s not too hard to figure out: Without special correction, a lens system will bring different wavelengths of light into focus on different planes. High-quality camera lenses are usually well-corrected apochromats, so this isn\'t a problem at the image sensor. The relatively simple optics (lenses and prisms) in AF systems are another matter, though, so it\'s very likely that long (red) wavelength light would produce a different focus point than would short (blue) wavelength illumination. By taking the color of the light source into account, the camera can compensate for this tendency, in much the same way that it can handle micro focus adjustments for different lenses.
Jan 13, 2013 at 11:26 PM
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