1 The Director of Photography – an overview

Lenses 59
Figure 5.6 Test shapes for lens
distortion
Figure 5.7 Pincushion distortion
Figure 5.8 Barrel distortion
rendition and feel of the Cooke zooms. They are also all optimized to
work at their common maximum aperture of T2.
Choice
So which range of lenses should you choose? It doesn’t really matter –
Ziess, Primos and Cooke S4s are all absolutely superb.What is important
is that you read the script first, decide on the look you want and then
choose the lens set that suits you best. Today’s cinematographers are
extraordinarily lucky to have a choice of at least three great sets of lenses.
Lens distortion and aberrations
The three most common forms of optical distortion that occur in lenses
are known as pincushion distortion, barrelling and chromatic aberration.
Prime lenses very rarely show any discernible distortion.The exception
might be extreme wide-angle lenses, some of which show a slight
tendency to barrel.
The finest zoom lenses will show no discernible distortion even at
the widest end of their range. Unfortunately, many current zoom lenses,
mainly those intended for or derived from those originally intended
for video cameras, still demonstrate considerable distortion. Again,
the most common or exaggerated distortion is barrelling at the wider
end of the zoom’s focal range. Because the market for video zooms
asks for very high zoom ratios coupled with wide maximum apertures,
the manufacturers cannot meet all the parameters of a perfect lens at
prices the market will bear.
The manufacturers do their best but, as in all things, you get what
you pay for and if you are asking for two of the lenses’ attributes to
be long zoom range and wide aperture, then almost certainly other
parameters of the design will suffer. In this case, the choice usually
tends towards a design where a noticeable amount of barrelling occurs
at the wide end and very noticeable image size change shows strongly
when the focus is changed.
In many applications barrelling and image shifts are not important
drawbacks, since the kind of images the lenses will be used to record
mask the faults. For news and sport, where the camera is rarely
absolutely still, any image shift when focusing will not be noticeable.
Slight barrelling will only show where there are vertical lines in the
scene near the edge of the frame and, again, this does not occur often
in these applications.
The effect of distortion
Figure 5.6 shows a series of shapes in their perfect form. There is
a square containing a circle that itself contains another square. Were
we to photograph this shape using a lens that suffered from pincushion
distortion, then the resultant image would look very much like
Figure 5.7, not at all acceptable.
If the same Figure 5.6 was photographed using a lens suffering from
barrelling, then the image created would appear as shown in Figure 5.8,
again unacceptable.