ABSTRACT
The main object of photographic equipment design and manufacture is to produce lenses
which minimize optical defects (known as ‘aberrations’) while achieving the highest possible
resolution of detail and image brightness. To achieve this a range of special optical glasses is
used, each type having different refraction and dispersion properties. A photographic lens has
a ‘compound’ construction, containing a series of elements of different shapes and made from
different glass types to help neutralize aberrations. In fact, a camera lens of normal focal length
typically has 5-8 elements (Figure 3.2). Their centring and spacing within the lens barrel is
critical, and can be upset if the lens is dropped or roughly knocked. However, even the number
of elements causes problems, as the tiny percentage of light reflected off every glass surface at
the point of refraction multiplies as scattered light. If uncorrected, the result would be images
that lack contrast and sparkle – like looking through a window with multiple double-glazing. The
best lenses therefore have their elements surface-coated with one or more extremely thin layers
of a transparent material that practically eliminates internal reflections under most conditions.
