1 The Director of Photography – an overview

Lenses 61
Figure 5.11 The Cooke triplet
Figure 5.12 The Zeiss Tessar
As glasses of many more densities became available and the value
of constructing more complicated lens groups was understood, it
became increasingly possible to arrange for more and more wavelengths
of light to focus at the same point after passing through the
whole of the lens. This was achieved by combining both positive and
negative lenses in groups, so that the failings of the individual elements
combined to cancel each other out, thus leaving the complete lens
aberration-free.
The first lenses that successfully corrected for the three colours of
the visible spectrum to converge at the same point of focus were
known as triplets, as they were constructed from three single elements
having differing curvatures and made from glasses of different densities.
Figure 5.11 shows one of the early, successful triplets, from the
firm of Cooke, Taylor, Taylor & Hobson, the company now known as
Cooke Lenses.
Further developments brought the Zeiss Tessar, as shown in Figure
5.12, where the rear lens of a basic triplet design has two simple lenses
cemented together for the rear element, thus enabling the optical
designer greater freedom of design.
Lenses have come a long way since the discovery of the basic triplet.
For example, the current Cooke S4 25 mm lens – the lens elements
here are in double figures and the mechanics of focusing are now
much more sophisticated than a simple screw thread. Lenses such as
this have eliminated all apparent distortions and aberrations, come in
perfectly matched sets and can be utterly relied upon.