This article details lenses for single-lens reflex and digital single-lens reflex cameras (SLRs and DSLRs respectively). The emphasis is on modern lenses for 35 mm film SLRs and for "full-frame" DSLRs with sensor sizes less than or equal to 35 mm.
Design
Interchangeable lenses
On most SLR and DSLR cameras, the lens can be changed. This enables the use of lenses that are best suited for a given photographic need, and allows for the attachment of specialized lenses. Film SLR cameras have existed since the late 1950s, and over the years, a very large number of different lenses have been produced, both by camera manufacturers (who typically only make lenses intended for their own camera bodies) and by third-party optics companies who may make lenses for several different camera lines.
DSLRs became affordable around the mid-1990s, and have become extremely popular in recent years. Some manufacturers, such as Minolta, Canon and Nikon, chose to make their DSLRs 100% compatible with their existing SLR lenses from the beginning, allowing owners of new DSLRs to continue to use their existing lenses and get a longer lifespan from their investment. Others, for example Olympus, opted for new lens mounts and series of lenses for their DSLRs. Modern Pentax DSLRs like the K-3 and K-50 are backwards-compatible with K-mount SLR lenses, though features that are included in newer lenses (e.g. autofocus) may not work. There are a few exceptions; the MZ and ZX series of Pentax film cameras do not work with some older lenses.
As implied by the above, lenses are only directly interchangeable within the mount system for which they are built. Mixing mounting systems requires an adapter, which most often results in compromises such as loss of functionality (e.g. lack of autofocus or automatic aperture control). Furthermore, the adapter may require an additional optical element to correct for varied registration distances (the distance from the rear of the mount to the focal plane on the image sensor or film). Adapters may not be available to bridge every combination of lens mount and camera mount.
Aperture and depth of field
The aperture of a lens is the opening that regulates the amount of light that passes through the lens. It is controlled by a diaphragm inside the lens, which is in turn controlled either manually or by the exposure circuitry in the camera body.
The relative aperture is specified as an f-number, the ratio of the lens focal length to its effective aperture diameter. A small f-number like 2.0 indicates a large aperture (more light passes through), while a large f-number like 22 indicates a small aperture (little light passes through). Aperture settings are usually not continuously variable; instead, the diaphragm has typically 5–10 discrete settings. The normal "full-stop" f-number scale for modern lenses is as follows: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, but many lenses also allow setting it to half-stop or third-stop increments. A "slow" lens (one that is not capable of passing a lot of light through) might have a maximum aperture from 5.6 to 11, while a "fast" lens (one that can pass more light through) might have a maximum aperture from 1 to 4. Fast lenses are, by definition, larger than slow lenses (for comparable focal length), and typically cost more.
The aperture affects not only the amount of light that passes through the lens, but also the depth of field of the resulting image: a larger aperture (a smaller f-number, e.g. f/2.0) will have a shallow depth of field, while a smaller aperture (a larger f-number, e.g. f/11) will have a greater depth of field.
Focal length and angle of view
The focal length of a lens, together with the size of the image sensor or film in the camera, determines the angle of view. A lens is considered to be a "normal lens" in terms of its angle of view on a camera when its focal length is approximately equal to the diagonal dimension of the film format or image sensor format. This diagonal angle of view—about 53 degrees—is often said to approximate the angle of view of the human eye; since the angle of view of a human eye is at least 140 degrees, it is sometimes noted as "similar to the angle of crisp human vision." A wide-angle lens has a shorter focal length and includes more of the viewed scene than a normal lens; a telephoto lens has a longer focal length and captures a small portion of the scene, making it seem closer.