Focus and recompose is a frequently used technique by many photographers. It is often used when you want to compose their subject in an area of the frame that doesn’t have an autofocus point nearby. On many cameras which have just a few focus points, focus and recompose is sometimes the only strategy that you will have at your disposal. When shooting in bright light with small apertures, this strategy can work just fine. However, when you shoot with wide apertures, focus and recompose can lead to poor results, even failure.

At the end of the post, he suggests PhotoCalc as one of the ways to calculate depth of field values while shooting in the field.