Definition of Telephoto Lens

A telephoto lens is any lens that contains a grouping of telephoto elements. This results in a lens with a long focal length, such that the lens itself is shorter than its own focal length. Telephoto lenses can be used to magnify distant subjects or to achieve zoom effects.

Development

Theories about the possibility of constructing a telephoto lens predate photography. In fact, many of the same optical principles are at work in telescopes that date back to the 17th century.
The telephoto lens was adapted for photography by Thomas Dallmeyer. His telephoto lens, built in 1891, resembled the modern telephoto lens. But Dallmeyer's status as "inventor" of the telephoto lens is a subject of debate, as other practitioners of the new medium of photography developed similar lenses at about the same time. Alexander McKay is notable for making his own telephoto lens out of the glass from discarded liquor bottles as early as the 1880s.

Optics

In a standard lens with a fixed focal length, the perspective and magnification are set and unchangeable. In a lens with varying focal lengths, the physical lengthening of the lens allows the photographer to magnify a distant subject by increasing the focal length.
In a telephoto lens, the telephoto group (a series of lens elements) is positioned close to the focal plane and distorts the light coming from lens elements farther away. This gives the illusion that the focal length is much longer than it actually is, longer even than the lens would be capable of producing if not for the presence of the telephoto group.

Variations

A retrofocus lens is essentially an inverted telephoto lens. It has the ability to make objects seem more distant using the same optical principles as the telephoto lens. Many modern lenses incorporate both the telephoto and retrofocus groupings to provide an extremely versatile zoom lens with a widely variable focal length.
Telephoto lenses (and their related zoom lens variants) have been incorporated into film cameras, and they are an important tool for photographers and cinematographers alike.

Effects

The basic purpose of the telephoto lens is magnification. This allows photographers to maintain a distance from a subject or to capture images in greater detail than their proximity would otherwise allow.
Other applications of the telephoto lens that might be explored by photographers include the manipulation of depth of field, which allows for only a portion of the image to remain in focus while objects in the distance or nearer the camera appear out of focus. The flattening of distance that is common to telephoto lenses can also be used to distort space in interesting ways or flatter a portrait subject by de-emphasizing sharp or elongated features.

Drawbacks

The chief complaint about the telephoto lens is its distortion of perspective. This phenomenon is known as "constant object size," and it refers to the compression of distance that is a function of the optics of a telephoto lens. It occurs because of the fact that as a telephoto lens is used to zoom in on a subject, the size of that subject relative to anything else in the frame remains constant instead of shifting due to a perspective shift. This shift could be achieved by physically changing the position of the camera, but this is not always possible due to spatial constraints.
When the zoom lens was popularized in the cinema, zooms were often criticized as unrealistic and conspicuous, but over time viewers became accustomed to their presence. In still photography, the old axiom that the camera never lies should at least be questioned in the case of perspective when a telephoto lens was used.

Resources

• Tamron.com: Fundamentals of Lenses