1 The Director of Photography – an overview

Testing 151
focus rings if they are using something like the Arri follow-focus
device. If this is to be done, then both left-hand and right-hand rings
need to be prepared.
Infinity test
It is important that all your lenses can focus as far as infinity. To test
this can be taxing as, other than with the iris wide open, there will be
sufficient depth of field for almost any lens to show infinity as sharp.
There is a difference of opinion on this test. Some focus pullers insist
on shooting this test, while others feel it is sufficient to eye-test the
lenses. My feeling is that an eye test is probably adequate. If you
decide to shoot a test then you have two options. If the camera you are
using has a variable shutter, then you can close this down to obtain the
correct exposure with the lens wide open. If the camera does not have
a variable shutter, then you will have to shoot at dusk or dawn.
You must not, under any circumstances, put neutral density filters in
front of the lens for this test. Putting glass filters in front of a lens will
alter the point at which the lens focuses on infinity. This is particularly
noticeable with lenses of long focal length. Indeed, it is essential that
all the longer lenses are infinity tested with and without a Wratten 85
filter to check they can reach infinity in both conditions. If the DP you
are working with tends to use a lot of filters, then test the longer lenses
with several filters on.
You will notice that with most long-focus lenses the focus barrel will
go past infinity. This is to ensure that you can reach infinity with several
filters in front of the lens, as the effect on infinity when adding
extra glass to the front of the lens is greater the longer the focal length
of the lens.
Resolution test
There are a number of wall charts available for lens testing and most
of them employ a series of parallel white lines on a black surface. The
lines are so arranged in blocks of finer and finer lines in order to discover
how fine the line must be before the lines cannot be seen apart.
This is the limit of the lens’s resolution. These measurements only
work if the chart exactly fills the frame.A good example of such a lens
test chart, designed and produced by KJM Consultants for Panavision,
can be seen in Figure 17.2. Do not copy and use this illustration from
the book – if reproduced in this way it will not have sufficiently fine
definition to give any meaningful results.
These days it is more common to test the finite resolution of a lens
on the bench either by measuring its MTF (modular transfer function)
or by projecting a very accurate chart, similar in appearance to that
described above, through the lens onto a wall where, due to the large
size of the projected image, the judgement of which are the finest lines
resolved can more easily be made.
The testing of the ultimate resolving power of a lens is best done in
conjunction with the specialist at your hire house. If you have worked
with the hire house before and trust them, then you can, quite reasonably,
ask them to have checked all the lenses before you start your
testing.