Page "Depth of field" Paragraph 3

In optics, particularly as it relates to film and photography, depth of field ( DOF ) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image.

Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.