1 The Director of Photography – an overview

152 Practical Cinematography
Figure 17.2 A lens test chart
Focus tests at various distances
Focus testing is time-consuming and must be done with great care. It
usually involves covering a wall of the test room with lens test charts,
such as that shown in Figure 17.2.This is so that the entire frame can be
filled with charts. An alternative, but more time-consuming, method is
to relocate a chart at the top and bottom edges of the frame, both sides
and all four corners. This is in addition to keeping one at the centre.
Mercifully, most hire houses have a wall of charts, or something similar,
already set up, but moving charts around is cheaper if you are having
to test independently.
As previously stated, a lens test chart can only be used for measurement
of resolution when it exactly fills the frame. With focus testing,
we are not looking for a measurement but are checking if the lens
focuses at the distance indicated on the lens barrel or, if it doesn’t,
making a new mark at that focus ourselves; secondly, we are checking
if the lens maintains the focus at the edges of the frame when it has
been focused at the middle of the frame.
For focus testing you are only using lens test charts as very easily
readable images from which to tell if the lens is focusing properly.
Each focus puller will have a preferred set of distances at which they
like to test and mark up the lenses. They may not always use the same
ones. For example, if you are about to shoot an intimate script, and are
on location, then the nearer focus points are more important, since that
is where your cast are going to be most of the time. On the other hand,
if you are going to shoot a western, you are going to spend some of the
time shooting close-ups but a lot of the shoot will be nearer infinity.
Many focus pullers simply abandon the distances engraved on the
lens and mark up their own with thin tape and a fine, waterproof
marker pen.
The technique is to set up the camera exactly at right angles to the
wall of test images and perfectly central to them. Every lens must then
be checked, through the viewfinder, at every distance the focus puller
requires.This does take a long time, but can save much grief on the set
during shooting. Some film must be exposed with each lens at each