7D low light AF... Canon vs Sigma std zoom

Started Aug 20, 2011 | Discussions thread
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dougr
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7D low light AF... Canon vs Sigma std zoom
Aug 20, 2011

Decided to sell my 15-85 IS and have decided on either a used Canon 17-55 2.8 IS, or a new Sigma 17-50 2.8 OS (hard to pass up a 4yr warranty and return privileges for the $50 difference between new and what they're bringing on the used market). Anyway, I had a number of things going on with the 15-85 that made it difficult to get a high percentage of keepers with my 7D. First, I prefer natural lighting (even where there is not much of it). Because of this, I often found myself above ISO 2500 where the noise becomes a bit bothersome... either that, or get motion blur due to too low shutter speed. Lastly, I have theorized that because of the max speed of the 15-85 being 3.5, I was missing out on the best focus performance (especially at close distances with shallow DOF) because the high precision center AF point requires at least an f2.8 max aperture. I was using single shot center AF point selected (just as I usually did for the same type of shot with my 20D and Sigma 18-50 2.8... with better success). Sooo... back to a 2.8 lens, but I would like anyone with experience with both lenses to give there impressions of low-light AF performance between these two. I've read professional reviews that have: 1) claimed the Canon is inconsistent with AF, 2) Claimed the Sigma is less accurate than the Canon, and 3) claimed the Sigma is at least as good as the Canon. IF the Sigma is comparable to the Canon in AF performance, the other differences are not enough for me to justify the price difference. Any help would be appreciated.

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