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A practical camera lens will most often incorporate an aperture adjustment mechanism, usually an iris diaphragm, to regulate the amount of light that may pass. In early camera models a rotating plate or slider with different sized holes was used. These Waterhouse stops may still be found on relatively modern, specialized lenses. A shutter, to regulate the time during which light may pass, may be incorporated within the lens assembly (for better quality imagery), or may be within the camera, or even, rarely, in front of the lens. Some cameras with leaf shutters in the lens omit the aperture, using the shutter to perform this function.

The lens may usually be focused by adjusting the distance from the lens assembly to the image-forming surface, or by moving elements within the lens assembly. Some lenses include a cam system which may vary the distance between the groups to provide better performance when the lens is focused at different distances. This system is usually referred to differently by different manufacturers for marketing purposes. Nikon calls this technology CRC (close range correction) while Hasselblad uses the term FLE (floating lens element)[1].

The lens elements are made of transparent materials. Glass is the most widely used material due to its good optical properties and resistance to scratching. Sometimes lenses are made of materials such as quartz or fluorite.[1][2][3] Various plastics, such as acrylic (the material of Plexiglass) can also be used. Occasionally, even germanium and meteoritic glass have been used.[4] Plastics allow the manufacture of strongly aspherical lens elements which are difficult or impossible to manufacture in glass, and which simplify or improve lens manufacture and performance. Plastics are not used for the outermost elements of all but the cheapest lenses as they scratch easily. Moulded plastic lenses have been used for the cheapest disposable cameras for many years, and have acquired a bad reputation: manufacturers of quality optics tend to use euphemisms such as "optical resin". However many modern, high performance (and high priced) lenses from popular manufacturers include molded or hybrid aspherical elements, so it is not true that all lenses with plastic elements are of low photographic quality.

The 1951 USAF Resolution Test Chart is one way to measure the resolving power of a lens in a standardized manner. The quality of the lens material, of the coatings and of the internal manufacture all influence the "resolution" of the lens, even at the same F-number and focal length. Lens resolution is limited by diffraction, and very few photographic lenses approach this resolution. Ones that approach this resolution with the aperture wide open are termed diffraction limited and are usually extremely expensive.[2] Today, most lenses are multi-coated in order to minimize lens flare and other unwanted effects. Some lenses have a UV coating to keep out the ultraviolet light that could expose photosensitive materials and result in inaccurate colors. Most modern optical cements which are used for bonding two glass elements together block UV light quite efficiently, which negates the need to use UV filters with most lenses. UV photographers will go to great lengths to find lenses that they can use effectively for their work, converting lenses with no cemented elements for possible UV work by polishing away the lens coatings, if present.

[edit] Aperture and focal length
Nikon's AF-S DX Zoom-Nikkor 18–70 f/3.5–4.5G IF-ED, an example of a lens with a zoom (focal length that can change), and a lens with a variable maximum aperture (the widest aperture changes as you zoom)
Nikon's AF-S DX Zoom-Nikkor 18–70 f/3.5–4.5G IF-ED, an example of a lens with a zoom (focal length that can change), and a lens with a variable maximum aperture (the widest aperture changes as you zoom)
The Canon 85mm f/1.8 is a compact lens popular with portrait photographers. Its large aperture can be used to minimize flash requirements or to produce a shallow depth of field.
The Canon 85mm f/1.8 is a compact lens popular with portrait photographers. Its large aperture can be used to minimize flash requirements or to produce a shallow depth of field.

The two main optical parameters of a photographic lens are the maximum aperture and the focal length. The focal length determines the angle of view, and the size of the image relative to that of the object, while the maximum aperture limits the brightness of the image and the fastest shutter speed usable. A popular third consideration is close focusing distance.

The maximum usable aperture of a lens is usually specified as the focal ratio or f-number, which is equal to the focal length divided by the effective aperture (or entrance pupil) diameter in the same units. The lower the number, the more light per unit area is delivered to the focal plane. Larger apertures (smaller f-numbers) provide a much shallower depth of field than smaller apertures, other conditions being equal. Practical lens assemblies may also contain mechanisms to deal with measuring light, secondary apertures for flare reduction, [3]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 marked in centimetres (cm) and inches are still to be found. For a given film or sensor size, specified by the length of the diagonal, a lens may be classified as

* Normal lens: angle of view of the diagonal about 50° and a focal length approximately equal to the diagonal produces this angle.
* Macro lens: angle of view narrower than 25° and focal length longer than normal. These lenses are used for close-ups, e.g., for images of the same size as the object. They usually feature a flat field as well, which means that the subject plane is exactly parallel with the film plane.
* Wide-angle lens: angle of view wider than 60° and focal length shorter than normal.
* Telephoto lens or long-focus lens: angle of view narrower and focal length longer than normal. A distinction is sometimes made between a long-focus lens and a true telephoto lens: the telephoto lens uses a telephoto group to be physically shorter than its focal length.

The 35mm film format is so prevalent that a 90mm lens, for example, is sometimes assumed to be a moderate telephoto; but for the 7×5cm format it is normal, while on the large 5×4 inch format it is a wide-angle. In general, the smaller the film or sensor surface, the smaller the angle of view. This can be corrected with lenses with shorter focal lengths.
An example of how lens choice affects angle of view. The photos below were taken by a 35 mm camera at a