The general rule is as stated above; the center AF point is accurate to 1/3 of the DOF when used with an f/2.8 or faster lens; the other AF points, like the center point with a lens slower than f/2.8, are accurate to within the DOF. Note that it does not matter what the aperture is set to for the picture - the aperture is wide open for focusing, then stops down just at you take the shot (which is why there's a DOF preview button!).
RE:"I do have 4 lenses at 2.8 or better, I just can't afford to have a longer lens faster than 4.0." - If you mean zoom lenses, yes - fast zooms are $$. Have you considered a prime? The 85mm f/1.8 is a great lens for portraits on a 1.6 crop body (136mm equivalent for 35mm, which is a 'classic' focal length for tight head shots, and works for full body shots with nice compression if you have a large space to work in). On a crop body like the XTi, a 50mm lens equates to 85mm full frame (a 'classic' portrait length for body shots) - Canon's 'nifty-fifty' (the 50mm f/1.8) is ~US$100, and is a great value and quite sharp lens (focus is accurate but loud and slowish, and bokeh is poor, but neither of those should matter much in a studio setting).
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Originally Posted by neuroanatomist
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