# 11. What is depth of field?

## Description

This article is from the
Photographic Lenses FAQ, by David Jacobson with numerous
contributions by others.

# 11. What is depth of field?

It is convenient to think of a rear depth of field and a front
depth of field. The rear depth of field is the distance from the
object to the farthest point that is sharp and the front depth of
field is the distance from the closest point that is sharp to the
object. Sometimes the term depth of field is used for the combination
of these two, i.e. the distance from the closest point that is sharp
to the farthest point that is sharp.

frontdepth = So - Sclose
frontdepth = Ne*c/(M^2 * (1 + (So-f)/h))
frontdepth = Ne*c/(M^2 * (1 + (N*c)/(f*M)))
frontdepth = So /(hr + 1)

reardepth = Sfar - So
reardepth = Ne*c/(M^2 * (1 - (So-f)/h))
reardepth = Ne*c/(M^2 * (1 - (N*c)/(f*M)))
reardepth = So/(hr - 1)

In the last three, if the denominator is zero or negative, reardepth is
infinity.

These formulas using hyperfocal distance can be used as
follows. Suppose I know that the object distance, So, is 1/8th of the
hyperfocal distance. Then the range of distances that is acceptably
sharp is from 8/9 of So to 8/7 of So. The front and rear depths of
field are 1/9 So and 1/7 So.

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