Photographic lens

A photographic lens (or more correctly, objective) is an integrated system comprising one or more simple optical lens elements, used for a camera or microscope. It may be fixed to the camera body, or interchangeable. There will usually be an aperture selection mechanism for adjusting the amount of light through the lens, and a focusing mechanism. Depending on type of camera, there may also be an integrated shutter. Objective lenses or lens systems are also used in playing and recording optical media, such as CDs.

The lens elements are made of a transparent material. Glass is the most popular, except for the lenses in the least-expensive cameras. Various plastics can substitute for glass, with acrylic (or PMMA), the material of Plexiglas, being one possibility. This is easier than glass to form into special shapes, such as surfaces that are not spherical, that have advantageous optical properties. However, it scratches easily, so glass is preferable for the outermost elements (which require occasional cleaning). For aspherical elements, plastics are common even in expensive photographic optics, although manufacturers often conceal this by substituting terms such as "optical resin."

The maximum aperture opening will normally be specified, as the f-number. The lower the number, the more light is admitted through the lens. On lenses for SLR cameras, there is usually an auto mechanism, where by the aperture is held fully open while focusing, but closes to its preset value when an image is taken.

The most important characteristic is the focal length. Nowadays, it is usually measured in millimeters (mm), but up to around 1960, the centimeter (cm) was common, and older used lenses with focal lengths marked in inches are still available. The focal length, in comparison to the film or sensor size (specifically, the diagonal measurement of the frame), places the lens in one of three categories:

• Wide-angle lens (with a focal length considerably smaller than the diagonal of the the frame)
• Normal lens (with a focal length about the same as the diagonal of the frame—give or take 20 or 25%)
• Telephoto lens (with a focal length considerably larger than the than the diagonal of the frame)

The comparison with frame size is important. An example can illustrate how it works. For large-format sheet film, measuring 4 by 5 inches (102 by 127 mm) and having a diagonal of about 160 mm, a 110 mm lens has a focal length quite a bit smaller than the diagonal. It is a wide-angle. On a 6×9 medium-format film frame, with a diagonal of about 105 mm, the same lens almost matches the diagonal, so it's a normal lens. And on 35 mm film, the focal length is much more than the frame diagonal (namely about 43 mm), so this is a telephoto.

In practice (i.e. when actually taking pictures,) this type of categorization is actually meaningless—as it will be shown further down in the article—, since the photographer chooses the lens and zoom factor based on practical reasons: a famous photographer said that the best lenses are your feet (i.e. move closer to the subject if practically possible, instead of "zooming" in.) However, the generic classification shown above is useful for several reasons:

• it's good semantics: people can talk about the generic "class" of lenses (e.g. "I had to use a telephoto, the squirrel was too far away);
• it's a good conventional way to classify lenses in shops: both the seller and the customer know what to expect (although this can be subjective for versatile zoom lenses, see below);
• even since versatile lenses appeared, the classification is meaningful semantically ("I had to shoot in telephoto, the squirrel was too far away".)

Some lenses can change their focal length. This is called zooming, and lenses capable of this are zoom lenses. Lenses with fixed focal length are called prime lenses. Zooms can be wide-angle, normal, or telephoto. Also, they can cross over from one category to another. Some zooms cover all three categories. Both for film and for digital photography, zoom lenses are very common in point-and-shoot cameras, and also in single-lens reflex cameras. They're less popular in medium-format photography, and there aren't any zooms for cameras that take the largest film sizes.

Prime lenses might have only three or four elements. Words like "triplet" or "Tessar" (which resembles the Greek tessera, meaning "four") in the name of a lens often hint at the number of elements. The most versatile zooms often have fifteen or more. The reflection of light between these individual pieces of glass or plastic seriously degraded the contrast and color saturation of early zoom lenses, especially in pictures that included the sun. Advances in coating technology have resulted in major improvements, and modern high-quality zooms yield crisp, colorful photographs.

Some notable photographic optical lens designs are:

• Angenieux retrofocus
• Cooke Triplet
• Double-Gauss
• Leitz Elmar
• Zeiss Tessar
• Zeiss Sonnar
• Zeiss Planar

Current photographic lenses manufacturers :

• Canon
• Konica Minolta
• Leica
• Nikon
• Pentax
• Tamron
• Tokina
• Schneider
• Sigma Corporation
• Zeiss

See also

• Lens (optics)
• teleconverterde:Objektiv (Optik)

fr:Objectif photographique ja:写真レンズ nl:Objectief (lens) pl:Obiektyw fotograficzny sv:Objektiv th:เลนส์ถ่ายภาพ zh:镜头