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I just revisited a book I have tonight (Rebel t2i Digital Field Guide by Charlotte Lowrie) I got it back when I first got my Rebel but it has been great to refer back to occasionally. I was thinking of selling it but wanted to re-read it before doing so. Anyways, it said this and it was very interesting:
According to many Canon documents, having autofocus set on One-Shot and recomposing the shot may cause the camera to back focus slightly and account for a loss of sharpness.
I do this quite a bit; I most often have it set on One Shot for portrait work of non-moving subjects as it’s said to offer the best for sharpness and since I like to be able to recompose my shot. But I’ve had instances where I knew I focused (using one of the 9 autofocus points) on the subject’s eyes yet some of my images don’t turn out as sharp. This is with generally shooting wide-open (I usually shoot at 2.8 which is the widest one of my lenses goes) and a pretty fast shutter speed, and adjust my ISO accordingly. So the chance for me having problems with camera shake are pretty minimal, yet sharpness is still an issue. Not all the time, but some of the time. This “discovery” could be the reason. I guess the solution, and one I’m going to practice, is to try to compose the shot how I’m going to press the shutter, but if I feel I don’t like the composition, I can crop it down later in post. Therefore I’d probably have to include more in the frame with the anticipation of trimming it down. That way I can keep one of the autofocus points right on the subject’s eyes (or wherever I plan on focusing). So I’m going to experiment a little. Has anyone heard of this?
And to revisit also some of the questions posted earlier, the closer you are to your subject when shooting wide-open, the smaller focal plane you’ll have available. Like if you’re standing a couple feet away their eyes might be in focus but the tip of their nose will not at 1.4 or even 2.0. If you’re 15 feet away, there is much more leeway. There will be a greater plane of focus.