Introduction

Camera lenses are optical systems designed to give a real image of distant objects. Therefore they must have a positive power with their aberrations corrected to produce reasonably good image quality for an object plane at infinity, that is for the image formed in the back focal plane of the lens. Some camera lenses known as “macro-lenses” are designed to give optimum image quality at intermediate or close object distances rather than for the object at infinity. Examples of macro-type lenses are those used in photocopiers and for photographing documents.

Camera lenses usually have a variable aperture stop, known as the diaphragm, whose diameter is varied within limits, for control of the image illuminance and hence film exposure (Section 21.4) and the depth-of-field (Section 21.6).

While a camera lens may consist of a single positive power lens, such a lens is not suitable when the focal length is very long or very short. Also a single lens would give less than adequate image quality for many applications, particularly those requiring large apertures or wide fields-of-view. Thus most camera lenses are complex optical systems, often consisting of two or more separate lenses. Designs suitable for very long and very short focal lengths are discussed in Section 21.2 and aberration considerations for wider apertures and field angles are discussed in Section 21.10.

Field-of-view, focal length and image sizes

The field-of-view of a camera system is usually limited by a field stop placed in the image or film plane. In 35 mm cameras, the field stop is 35 × 24 mm; thus the image or frame size on the film has these dimensions.