1 The Director of Photography – an overview

60 Practical Cinematography
Figure 5.9 Cropped barrel distortion
Figure 5.10 Chromatic aberration
If either of these distortions are present in the lens in use, the degree
of distortion can be measured, but more important is the perceived
degree of distortion. You have to look through the viewfinder and
decide if you can tolerate the effect you see.
The perceived level of distortion is usually a little less than one might
expect, as we are nearly always recording at the film plane a rectangular
image taken from the centre of the circular image formed by the lens.
Figure 5.9 shows exactly the same drawing as Figure 5.8, but cut down
to a 16 � 9 image format.
Although the distortion still looks unacceptable, it now only
appears at its worst at the sides of the frame; the overall effect is less
to the eye than in Figure 5.8. If you suspect a lens of barrelling then
frame a vertical line, say the side of a building, at the edge of the frame
and make your own judgement.
Chromatic aberration
Chromatic aberration is the most commonly found aberration in older
lenses. It is rarely found in modern lenses, though one occasionally
finds it in lenses of long focal length. It is worth understanding, as it
displays very clearly one of the major problems lens designers have to
overcome; indeed, the correction of chromatic aberration was the first
major optical breakthrough in the early days of photography.
Chromatic aberration occurs because light with different wavelengths,
or colours, will be diffracted, or bent, by a different amount
after having passed through the air/glass, glass/air path as the light
travels through a lens.
Figure 5.10 shows, in an exaggerated form, the effect of light passing
through a simple lens. The most deviated wavelengths are ultraviolet,
shown as UV in the diagram. The least deviated, and therefore those
coming to a point of focus furthest from the lens, are the infrared rays,
shown as IR in the diagram. In between, at focus points progressively
further away from the lens, are blue, green and red.
This effect of focusing the rays of differing colours at different distances
from the lens occurs in all simple, single lenses. It has long been
a well-known phenomenon: Sir Isaac Newton believed that it was such
an intractable problem that he concentrated on reflecting or mirror
optics for his telescopes, where the problem does not occur as the light
rays never pass through anything but air, thus obviating the problem.
White
Light
Optical
Centre
UV B G R IR