LENSES

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

The telephoto lens is one with a longer focal length than the normal lens and has a field of view that is narrower than the normal lens.
For example, the angle of view of a 200mm telephoto lens is about 12°, and 300mm lens has about an 8° view, compared to 46° for a 50mm lens.

Typical 300mm f4, Telephoto lens.

135mm f2 telephoto lens.

Telephoto lenses have focal lengths of between 75 and 1000mm or more. They have the effect of magnifying the size of the image on the film plane, so they include much less of the scene in the image than the normal lens when used at the same camera position. This gives the impression of compressing the separation between objects.
Telephoto lenses have a reduced depth of field (DoF) and will be in focus over a a restricted range for a given aperture, and as a result focussing is more critical than with a normal lens, especially with lenses in excess of 200mm focal length.
The shallow DoF can be used to advantage with some subjects, such as portraiture, when the background can be thrown out of focus while keeping the subject sharp. Similarly, when photographing plants, the grass and background clutter will be out of focus making the plant stand out sharply in the foreground.
The telephoto lens is useful when working in areas when one cannot get close to the subject or where it would be unsafe, such as when photographing wild animals or in dangerous situations, like motor racing events etc.

Where possible a tripod should be used, as camera movement is magnified and can cause images to be blurred, a monopod can be also be useful, especially in crowded areas.
If the camera has to be hand held then a fast shutter speed should be used.
A good guide to the slowest practical hand held shutter speed, is to use the reciprocal of the focal length i.e for a 300mm lens, 1/300th sec (1/250 on most cameras), a 500mm lens 1/500th and so on.
If using slower shutter speeds then the camera should be steadied on something solid, such as a wall or fence, to provide stability.

The picture on the left is a typical telephoto shot.
Taken with a 125mm lens it gives a magnified view of the scene. The use of a small aperture to ensure adequate DoF was important to ensure that both the foreground and background of the image are in focus.
The right hand picture was taken with a 300mm lens so as to frame the subject and remove unwanted objects. A large aperture was used to put the background out of focus.

Photographs taken with a telephoto lens will have an restricted angle of view, and can be used to create an apparent distortion of perpective. This can be used to good effect in some cases as it tends to make the the parts of the subject near to the camera appear smaller relative to those further away.

Compression effect
It is often said that a long focus lens compresses perspective.
This is not strictly correct, the compression effect is an illusion caused by the narrow field of view of the lens Click here for more info.

Telephoto lenses, tend to have smaller maximum apertures than normal lenses, between F4.0 and f5.6, so exposure times tend to be longer.
Larger aperture lenses are available but are relatively expensive and are used more by professionals or dedicated natural history enthusiasts.
The shallow DoF means that a small aperture is often used, resulting in longer exposure times, this can be overcome by using faster films, 400 ISO(ASA) or higher. The problem of the larger grain size of the faster film tends to be minimised by the negative requiring less enlargement, due to the larger image size on the negative.
Some modern telephoto lenses have macro capability, making them useful for close up work but the DoF can limit their scope if a slow fine grain film is used.

The diagram, left, shows the depth of field for
a typical 70 - 300mm lens at 70 & 300mmmm
settings & various apertures when focussed
on a subject at 15 and 250metres respectively

Note the variation in DoF at different settings of
the lens

Conclusion.

Advantages,
The telephoto lens is a useful lens for those situations where access would be difficult or impossible with a normal lens, or where the photographer needs to increase the size of the image on the negative.
It is a useful lens for portraiture, especially in the shorter ranges, 75 - 200mm, as it allows the photographer to utilize the short DoF to put the background out of focus and place the emphasis on the subject.
The illusion of image compression can often create interesting or unusual images.

Disadvantges,
Its narrow DoF can be a problem in low light, making it necessary to use a fast film, with the resultant increase in grain. This may be offset by not having to produce a greater enlargement of part of the negative if a normal or wide-angle lens is used.
To avoid blur caused by camera movement a tripod is often required.