Telephoto lens

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500 mm telephoto lens with extension tube.

In photography and cinematography, a telephoto lens is a specific construction of a long focal length photographic lens that places its optical centre outside of its physical construction, such that the entire lens assembly is between the optical centre and the focal plane. A regular lens of a focal length that is longer than what is considered a normal lens is not necessarily a telephoto lens. A telephoto lens has to incorporate a special lens group known as a telephoto group. (See below.) The angle of view created by a telephoto lens is the same as that created by an ordinary lens of the same specified focal length.

1 Construction
2 Effects
- 2.1 Still photographer
- 2.2 Constant object size
3 See also
4 References
5 External links

[edit] Construction

Cross-section - typical telephoto lens.

If a camera lens were to be constructed from a single lens of 200 mm focal length, then when the lens is focussed on an object at infinity, the lens will be 200 mm away from the focal plane where the film or sensor is. The centre of the lens is referred to as the optical centre of the lens. Even constructing the lens out of multiple elements in the regular fashion, usually to eliminate aberrations, will still have the optical centre within the construction.

As the focal length of such lenses increases the physical length of lens becomes inconveniently long. But such lenses are not telephoto lenses, no matter how extreme the focal length. They are simply known as long focal length lenses.

The diagram to the right shows the basic construction of a telephoto lens. It consists of front lens elements that, as a group, have a positive focus. The focal length of this group is shorter than the effective focal length of the lens. The converging rays from this group are intercepted by the rear lens group, sometimes called the "telephoto group," which has a negative focus. The simplest telephoto designs could consist of one element in each group, but in practice, more than one element is used in each group to correct for various aberrations. The combination of these two groups produces a lens assembly that is physically shorter than a long focus lens producing the same image size.

This same property is achieved with mirrors combined with lenses in catadioptric designs. The mirrors in such designs fold the light path and the curved secondary extends the light cone, making the lens much shorter than the focal length even given the folded design. However, lenses incorporating mirrors are not necessarily of telephoto design.

Compare with the opposite effect used in retrofocus lenses, sometimes described as inverted telephotos, which have greater clearance from the rear element to the film plane than their focal length would permit with a conventional wide-angle lens optical design. It is possible to construct zoom lenses that are telephotos at one extreme of the zoom range and retrofocus at the other.

The biggest telephoto is a 1700 mm f/4, implying a 425 mm exit pupil made by Carl Zeiss, mounted for an Hasselblad 203 FE and weighting 256 kg.^[1]

[edit] Effects

Telephoto lenses are best known for making distant objects appear magnified. This effect is similar to moving closer to the object, but is not the same, since perspective is a function solely of viewing distance. Two images taken from the same location, one with a wide angle lens and the other with a telephoto lens, will show identical perspective, in that near and far objects appear the same relative size to each other.^[2] However, the actual appearance is such that the telephoto lens compresses the distance between objects. Telephoto lenses also have less depth of field at a given aperture than shorter lenses and so require more precise focusing.

[edit] Still photographer

Effect of different focal lengths on photographs taken from the same place:

28 mm

50 mm

70 mm

210 mm

The photos above were taken by a 35 mm camera, using lenses of the given focal lengths.

[edit] Constant object size

One object photographed through 6 different focal lengths. The photographer moves as to keep the same angular size on the film. Observe that although the foreground object remains the same size, the background changes size; thus, perspective is dependent on the distance between the photographer and the subject. The longer focus lenses compress the perception of depth, and the shorter focus exaggerate it.^[3] The perspective is considered to be correctly represented by a 50 mm lens (for the standard 35 mm film format).