There's a (practically imperceptible) drain on battery life to have IS on, but the only real reason you'd want to turn IS off is if you had your camera mounted on a tripod. It will look for, and try to compensate for movement that isn't there sometimes and can actually create movement in that case.
Im looking at getting a new telephoto lens for my XTi. Would i get the same results with a 75-300mm USM III and a tripod that i would get with a 70-300mm USM IS? is IS just to reduce camera shake when u can't use a tripod? Should i just be looking at getting a non IS and a tripod and save a couple hundred?
the optics aren't very good in the 75-300 either. i'd go 70-200mm f/4L or 70-300 IS.
40d | 5d mk II | 2.8/16 zenitar fisheye | 16-35L | 35L | sigma 1.4/50 | sigma 2.8/50 Macro | sigma 1.4/85 | 70-200L IS website
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