Focus distance (Depth of field)
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.
The f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens’s focal length to the diameter of the entrance pupil. The required f-number is determined by finding the markings on the DOF scale that are closest to the near and far distance marks
For given near Dn and far Df Depth of Field limits the focus distance is smallest when focus is the harmonic mean of the near and far distances.
|s||Focus distance (mm)|
|Dn||Near depth of field (mm)|
|Df||Far depth of field (mm)|