see also lens
Depth of field is a measure of the zone of distances (from near to far) that are within acceptable sharpness at a given aperture and focus distance.
When a lens is focused on a subject at a given distance, all subjects at that distance are sharply focused. In theory, subjects that are not at the same distance are out of focus, and thus not sharp. However, since human eyes cannot distinguish very small degree of unsharpness, some subjects that are in front of and behind the sharply focused subjects may still appear to be sharp. The zone of acceptable sharpness is referred to as the depth of field. Thus, increasing the depth of field increases the perceived sharpness of an image. Using smaller apertures increases the depth of field.
Generally, depth of focus extends twice as far behind the point of focus as it does in front of it. So for a given aperture, focussing on a subject 15 feet away might allow items between 10 and 25 feet away to be acceptably focused -- 5 feet in front of the point of focus and 10 feet behind it.
- Depth of field explained by Ching-Kuang Shene