You've got to be careful regarding the combination of depth of field with "focus & recompose" techniques... and how much you move the frame. Suppose you've got a fairly decent depth of field... enough to get a group of people all in focus. Well then focus & re-compose at will... your depth of field will take care of it. But suppose you've got a really shallow depth of field... your subject's "eyes" are in optimal focus... but possibly you notice that even their nose or ears are not quite as tack sharp. NOW you've got to be careful. A small adjustment will alter your focus plane and the subject's eyes are no longer in that plane of tack-sharp focus. Modern lenses have loads of extra elements in them to correct for all sorts of optical issues and one of their jobs is to attempt to "flatten" the plane of focus (it would otherwise be moderately spherical). They usually do a pretty decent job... but not a perfect job. Here's a graphic that I drew up to illustrate the risk of focus & recompose. In this drawing, the camera is in the exact center of the blue circle. I was careful to draw a perfectly round circle (it's not an ellipse). The perimeter of the blue circle represents the focus distance. Everything on that circle is the identical distance from the camera. Notice the subject (and I drew a small thin line to denote the approximate position of our subject's eyes) is at the distance for perfect focus. I've done a gradient color on the rectangles (and made them slightly transparent) so you can see the depth of field... from good/acceptable (green) to poor/unacceptable (red). There are TWO of these colored rectangles. #1 The one located over our subject is the rectangle that we used to establish "focus". #2 The one located at the top of our graphic is the one we use for "composition" (the one we plan to use when we take the shot). After we focus on the eyes... we re-point the camera so that our subject will be at the right side of the frame (e.g. "rule of thirds" composition). Although the eyes were at the perfect focus distance in depth-of-field rectangle #1... that subject's eyes will clearly NOT be in acceptable focus in rectangle #2. This is because we used a shallow depth of field. If you took this shot at f/8... or f/11... this probably wouldn't matter. But if you're shooting this at f/2... or f/1.4... you may not be happy with the result.