Telephoto lens

"Telephoto" redirects here. For the technique for transmitting images, see Wirephoto.

Example of a 500 mm lens that is not a telephoto lens.

In photography and cinematography, a telephoto lens is a specific construction of a long focal length photographic lens in which the physical length of the lens is shorter than the focal length. In these lenses the optical center lies outside of its physical construction, such that the entire lens assembly is between the optical center 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); nevertheless, non-telephoto lenses of long focal length are often informally referred to as telephoto lenses. 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.

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 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 several elements to minimize 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. A telephoto lens works by having the outermost (i.e. light gathering) element of a much shorter focal length that the equivalent long-focus lens and then incorporating a second set of elements close to the film or sensor plane that extend the cone of light so that it appears to have come from a lens of much greater focal length.

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 f/4, 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).

Effects

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.

Still photography

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

• 
  28 mm
• 
  50 mm
• 
  70 mm
• 
  210 mm

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

Constant object size

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 pleasing perspective for portraits.

• 
  28 mm
• 
  50 mm
• 
  135 mm

History

The concept of the telephoto lens, in reflecting form, was first described by Johannes Kepler in his Dioptrice of 1611 [1], and re-invented by Peter Barlow in 1834. [2]

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. [3]

However, on the other side of the world in New Zealand, Alexander McKay was taking photographs of exceptional quality using home-made telephoto lenses, (ground from the bottoms of whisky bottles), probably as early as 1883 or 1884. Some of his photographs are preserved in the holdings of the Turnbull Library in Wellington, and two of these can be unequivocally dated as having been taken during May 1886.

One of McKay’s photographs shows the Russian warship Vjestnik anchored in Wellington harbour about two and a half kilometres away, with its rigging lines and gun ports clearly visible. The other, taken from the same point, is of a local hotel, the Shepherds Arms, about 100 metres distant from the camera. The masts of the Vjestnik are visible in the background. These photographs and documentation of McKay's other photographic achievements (photo-micrographs, and a ‘shadow-less technique’ for photographing fossils), are contained in the book, The Real McKay'[4] (pages 12, 17-19, 70-171, and 176.)

On 13th August, 1890, McKay presented a paper to the Wellington Philosophical Society (the precursor of the Royal Society of New Zealand) ‘On Some Means for increasing the Scale of Photographic Lenses and the Use of Telescopic Powers in Connection with an Ordinary Camera.’ His address was greeted with great enthusiasm and comments such as ‘will completely revolutionize photography' and ‘a world first.’ Sir James Hector noted that McKay had ‘perfected his invention after years of work and at great expense.’

See also

• Film format
• Secret photography
• Photographic lens design

External links

• Information on Catadioptric mirror lenses
• Further clarification: Why Telephoto?
• 3 part series on Cheap Super Telephoto Lenses (300-500mm)