1 The Director of Photography – an overview

Depth of field 141
Figure 16.7 Depth of field for a
given circle of confusion
Figure 16.8 How focal length
affects depth of field
Depth of field
Depth of field
start to be considered sharp when these rays have come close enough
together to be inside the hard lines.
The rays that will form the near point cross over inside all the other
lines beyond the lens, and it is only when they diverge and cross the
hard lines that they cease to be sharp, having formed the far point of
focus at the crossing point.
Figure 16.8 has been drawn in the same proportions as Figure 16.7,
but represents a 25 mm f2 lens. Two things change as a result of this
new lens: the film plane halves its distance from the centre of the lens
and the diameter of the lens is smaller. The ray diagram is drawn with
exactly the same parameters as Figure 16.7, but due to the new
proportions of the lens the rays forming the far point cross the hard
lines much nearer the lens and the rays forming the near point do not
diverge and cross the hard lines until much further away.Thus, a much
greater depth of field is created.
The mathematics of depth of field
50 mm lens at f2
25 mm lens at f2
Film
Plane
It is never necessary to go back to first principles and do the mathematics
involved on the set. Commercially available calculators are
very reliable and are to be trusted.
Before displaying the mathematics involved in computing the depth of
field in any given situation, the idea of hyperfocal distance must be understood.
Knowing the hyperfocal distance can be useful in practice, for it
tells us the maximum depth of field starting with infinity and coming back