Depth of Field "In optics, particularly film and photography, the depth of field (DOF) is the distance in front of and behind the subject which appears to be in focus. For any given lens setting, there is only one distance at which a subject is precisely in focus, but focus falls off gradually on either side of that distance, so there is a region in which the blurring is tolerable. This region is greater behind the point of focus than it is in front, as the angle of the light rays change more rapidly; they approach being parallel with increasing distance."

Hyperfocal Distance "The hyperfocal distance is the nearest distance at which the far end of the depth of field stretches to infinity. Focusing the camera at the hyperfocal distance results in the largest possible depth of field. Focusing beyond the hyperfocal distance does not add depth of field to the far end (which is already at infinity), but it does subtract from the focus area in front of the hyperfocal point. Therefore there is less total depth of field. Likewise, focusing ahead of the hyperfocal distance results in a gain of focus area ahead of the focus point but loses some of the focus area beyond the focus point including the subjects near infinity. Of course, this latter approach may be appropriate for images that do not extend to infinity." - Wikipedia