If a camera lens were to be constructed from a single lens of 200 mm focal length, then when the lens is focused on an object at infinity, the lens will be 200 mm away from the focal plane where the film or sensor is. The center of the lens is referred to as the optical center of the lens. Even constructing the lens out of multiple elements in the regular fashion, usually to eliminate aberrations, will still have the optical center 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. Zoom lenses that are telephotos at one extreme of the zoom range and retrofocus at the other are now common.
The heaviest telephoto lens was made by Carl Zeiss and has a focal length of 1700 mm with a maximum aperture of , implying a 425 mm (16.7 inch) entrance pupil. It is designed for use with a medium format Hasselblad 203 FE camera and weighs 256 kg (564 lb).
Telephoto and other long-focal-length 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 location. 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. Comparing magnification by using a long lens to magnification by moving closer, however, the telephoto shot appears to compress the distance between objects due to the perspective from the more distant location. Long lenses thus give a photographer an alternative to the type of perspective distortion exhibited by shorter focal length lenses where (when the photographer stands closer to the given subject) different portions of a subject in a photograph can appear out of proportion to each other.
Long lenses also make it easier to blur the background more, even when the depth of field is the same; photographers will sometimes use this effect to defocus the background in an image to "separate" it from the subject.
The photographer often moves to keep the same image size on the film for a particular object. Observe in the comparison images below 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. The perspective of the so-called normal lens, 50mm focal length for 35 mm film format, is conventionally regarded as a "correct" perspective, though a longer lens is usually preferred for a more correct perspective for portraits.
Histories of photography usually credit Thomas Rudolphus Dallmeyer with the invention of the photographic telephoto lens in 1891, though it was independently invented by others about the same time; some credit his father John Henry Dallmeyer in 1860.
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