LENSES

Note: The information in this section refers mainly to the 35mm format.

A normal lens is one that has a field of view that is similar to that of the human eye, and is approximately equal to the diagonal dimension of the negative format, 50-55mm for 35mm format 90mm for a 6 x 6cm negative etc.
Photographs taken with a normal lens will have a 'natural' perspective. Any objects in the scene will appear in the 'correct' proportions to each other.
The normal lens is ideal for general photography as it has a fair depth of field and can be focussed over a wide range from about 1metre (3ft) to infinity.

Wide angle lenses have focal lengths of between 15 and 35mm. Because they have the affect of reducing the size of the subject on the film plane they include more of the scene in the image than the normal lens when used at the same camera position.
Wide angle lenses also appear to exaggerate the size of objects close to the camera relative to more distant objects.
The depth of field of a wide angle lens is greater for a given aperture than a normal lens, which makes focussing less critical.
Wide angle lenses of less than 30mm focal length often cause straight lines to be slightly curved but modern aspherical lenses reduce or eliminate this problem.
Lenses with focal lengths less than 15mm (commonly known as fish-eye lenses) produce circular images with up to 220° angle of view. The image exhibits extreme distortion and every straight line in the image appears curved.

Lenses of 70mm focal length or more are termed telephoto lenses. They have a narrow field of view, for instance a 300mm lens will have an angle of view of about 8°. They magnify the subject in a similar manner to a telescope, thus making the subject appear closer and producing a larger image on the negative.
One effect of telephoto lenses is that they appear to increase the size of more distant objects relative to the nearer ones. This creates the illusion of squashing the perspective and squeezing objects together.
These lenses are useful for situations where one cannot get close to the subject, either for reasons of safety or accessibility, such as photographing wild animals or fast cars on a race track.
Depth of field is relatively small so accurate focussing is required.
With very long focal lengths (over 200mm) a tripod is essential, as every slight movement of the camera is exaggerated and will result in blurred images, unless fast shutter speeds are employed.

Zoom lenses are very complex lenses that allow one to vary the focal length within certain limits depending on the lens.
Common zoom ranges are 28-80mm, 70-300mm, and lenses with ranges of 28-300mm or more are available.
Zoom lenses are useful for framing a subject so that it fills the frame, and for eliminating unwanted or intrusive objects in the scene.
Depth of field varies depending on the focal length and aperture being used.

FIELD OF VIEW

The panoramic image below shows the relationship between the focal length of a lens and its field of view. The hatched blue rectangle represents the approximate view covered by the normal 50mm focal length lens of a 35mm camera. A long focus lens of 1200 mm focal length covers an area of about 2°, whereas a wide angle, 28mm lens covers around 75°. The ultra wide angle lenses, commonly called 'fish-eye lenses' can encompass angles of 180° or more.

Note: The angle of view for wide angle lenses also extends vertically, but is not shown in this diagram.