Camera lens
A lens might be permanently fixed to a camera, or it might be interchangeable with lenses of different focal lengths, apertures, and other properties.
A first step is to put a simple convex lens at the pinhole with a focal length equal to the distance to the film plane (assuming the camera will take pictures of distant objects[1]).
To improve performance, some lenses have a cam system that adjusts the distance between the groups as the lens is focused.
[2] Glass is the most common material used to construct lens elements, due to its good optical properties and resistance to scratching.
[citation needed] The 1951 USAF resolution test chart is one way to measure the resolving power of a lens.
[7] Today, most lenses are multi-coated in order to minimize lens flare and other unwanted effects.
A lens will most often have an aperture adjustment mechanism, usually an iris diaphragm, to regulate the amount of light that passes.
A wider aperture, identified by a smaller f-number, allows using a faster shutter speed for the same exposure.
Practical lens assemblies may also contain mechanisms to deal with measuring light, secondary apertures for flare reduction,[9] and mechanisms to hold the aperture open until the instant of exposure to allow SLR cameras to focus with a brighter image with shallower depth of field, theoretically allowing better focus accuracy.
Focal lengths are usually specified in millimetres (mm), but older lenses might be marked in centimetres (cm) or inches.
Photographs can be taken of a person stretching out a hand with a wideangle, a normal lens, and a telephoto, which contain exactly the same image size by changing the distance from the subject.
A moderate long-focus (telephoto) lens is often recommended for portraiture because the perspective corresponding to the longer shooting distance is considered to look more flattering.
The widest aperture lens in history of photography is believed to be the Carl Zeiss Planar 50mm f/0.7,[11] which was designed and made specifically for the NASA Apollo lunar program to capture the far side of the Moon in 1966.
The complexity of a lens — the number of elements and their degree of asphericity — depends upon the angle of view, the maximum aperture, and intended price point, among other variables.
A long-focus lens of small aperture can be of very simple construction to attain comparable image quality: a doublet (two elements) will often suffice.
Some older cameras were fitted with convertible lenses (German: Satzobjektiv) of normal focal length.
The simpler half-lens was of adequate quality for the narrow angle of view and small relative aperture.
Good-quality lenses with maximum aperture no greater than f/2.8 and fixed, normal, focal length need at least three (triplet) or four elements (the trade name "Tessar" derives from the Greek tessera, meaning "four").
The reflection of light at each of the many interfaces between different optical media (air, glass, plastic) seriously degraded the contrast and color saturation of early lenses, particularly zoom lenses, especially where the lens was directly illuminated by a light source.
A macro lens used in macro or "close-up" photography (not to be confused with the compositional term close up) is any lens that produces an image on the focal plane (i.e., film or a digital sensor) that is one quarter of life size (1:4) to the same size (1:1) as the subject being imaged.
Depending on the camera to subject distance and aperture, the depth-of-field can be very narrow, limiting the linear depth of the area that will be in focus.
