III.

Lens Measurement

The optical characteristics of simple, or single, lenses, or compound lenses (lens systems containing two or more individual elements), are determined by two factors: the focal length of the lens, and the ratio of the focal length to the diameter. Focal length is measured in two ways: in ordinary units of length, as for example 20 cm (8 in); or in units called dioptres, equal to the reciprocal of the focal distance measured in metres. A 1-dioptre lens has a focal length of 1 m (3 ft 4 in); a 2-dioptre lens has a focal length of 0.5 m (1 ft 8 in). The ratio of the focal length to the diameter of a lens determines its light-gathering power or “speed”. This ratio is the so-called f-number of the lens.