While you are shooting and concentrating on the action, the available depth of field can vary without your realizing it. So be prepared! Remember, when you refocus as the distance of the subject or camera alters, or you operate the zoom, or you change the f-stop (e.g. opening it up as you move into a building), the depth of field will alter! You will notice this most on closer shots.
As a rough guide, about one-third of the focused depth extends in front of the focused distance (focused plane) and two-thirds beyond it. The overall depth available at a given stop is greater for smaller CCD formats.
Most lenses perform best when they are only partially stopped down (e.g. f/5.6 to f/8). Although ...
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