# Depth of field

The area of sharpness in front of and behind the subject in perfect focus. Anything within this ‘depth of field’ will appear sharp.

1. The range of distance in subject space within which a lens (or a system) provides an image that reproduces detail with an acceptably small circle of confusion, usually small enough for subjective evaluation as a ‘point’ — defines the depth of field. Tables are calculated for lenses as a function of optical aperture and the subject distance at which they are focused. Regrettably, these calculations are strictly geometric (ignoring the possibility of diffraction effects, of all optical aberrations, and of possible differing contributions to focal length from different annuli of the optical system). Thus, the tables are at times overly optimistic.

2. Depth of field for a given imaging system decreases with increasing optical aperture of that system, and decreases as the distance to the subject decreases.

3. A ‘maximum acceptable’ diameter for the ‘circle of confusion’ may depend upon the resolution capabilities of the light-sensitive receptor (electronic or photographic) and of the system within which it is functioning.

Permissible Circle of Confusion for Video Lenses

• 1” = 0.03mm
• 2/3” = 0.021mm
• 1/2”=0.016mm
• 1/3” = 0.011mm
• 1/4” = 0.008mm

4. Quantitive measurements for actual imaging systems may be made on an optical bench, practical determinations are made from subjective examination of the actual images in the system of interest.To calculate the depth of field you mus first work out the ‘Far Limit of Depth’ and the ‘Near Limit of Depth’. The following formulas are used:

Far Limit (d 1 ) = Circ of C×f No.×t 2 / F 2 Cof C×f No.×t

Near Limit (d 2 ) = Circ of C×f No.×t 2 / F 2+ Cof C×f No.×t

C of C = Permissible Circle of Confusionf No. = Aperture numberF = Focal Lengtht = Distance focused upon.

Depth of Field = d 1 + d 2

Generally:

• Larger f-numbers give greater depth of field. (The more the iris is stopped down the greater the ‘depth of field’.
• Shorter focal lengths give greater ‘depth of field’.
• Greater subject distances gives greater ‘depth of field’.
• Depth of field is greater behind the subject than the front.