This about photographic lenses for single-lens reflex film cameras (SLRs) and digital single-lens reflex cameras (DSLRs).
Furthermore, the emphasis is on modern lenses for 35 mm film SLRs and for DSLRs with sensor sizes less than or equal to 35 mm ("full-frame").
(40-150 mm, 11-22 mm and 14-54 mm) and a Sigma prime (30 mm), all for the Four Thirds System.
Interchangeable lenses
The major advantage of SLR and DSLR cameras is the possibility of changing lenses, to select the best lens for the current photographic need, and to allow 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 available around the mid-1990s, and have become extremely popular in recent years. Some manufacturers, for example Minolta, Canon and Nikon, chose to make their DSLRs 100% compatible with their existing SLR lenses in the beginning, allowing owners of new DSLR's to continue to use their existing lenses and get a longer lifespan from their investment. Others, for example Olympus, chose to create a completely new lens mount and series of lenses for their DSLRs. The Pentax SLR camera K-mount system is backward compatible to all previous lens generations from Pentax, including the latest digital SLRs like the K-5 and K-r. A Pentax K-mount lens from the early 70s can be utilized on the newest Pentax DSLR. There are a few exceptions from the MZ and ZX series of Pentax film cameras that do not work with some of the 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, and most often results in compromises such as loss of functionality, i.e. auto focus or automatic aperture control. Further, in some cases the adapter will 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). Additionally, there are instances where an adapter is not available.
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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 f/2.0 indicates a large aperture (more light passes through), while a large f-number like f/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 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 typically 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 will have a shallow depth of field, while a smaller aperture 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 in the camera (or size of the 35 mm film), 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 thefilm format or image sensor format.[4] The resulting diagonal angle of view of about 53 degrees is often said to approximate the angle of human vision; since the angle of view of a human eye is at least 140 degrees,more careful authors will qualify that, for example 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 images a small portion of the scene, making it seem closer.
Lenses are not labeled or sold according to their angle of view, but rather by their focal length, usually expressed in millimeters. But this specification is insufficient to compare lenses for different cameras because field of view also depends on the sensor size. For example, a 50 mm lens mounted on a Nikon D3 (a full-frame camera) provides approximately the same field of view as a 32 mm lens mounted on a Sony α 100 (an APS-C camera). Conversely, the same lens can produce different fields of view when mounted on different cameras. For example, a 35 mm lens mounted on a Canon EOS 5D (full-frame) provides a slightly wide-angle view, while the same lens mounted on a Canon EOS 400D (APS-C) provides a "normal" or slightly telephoto view.
In order to make it easier to compare lens–camera pairs, it is common to talk about their 35 mm equivalent focal length. For example, when talking about a 14 mm lens for a Four Thirds Systemcamera, one would not only indicate that it had a focal length of 14 mm, but also that its "35 mm equivalent focal length" is 28 mm. This way of talking about lenses is not just limited to SLR and DSLR lenses; it is very common to see this focal length equivalency in the specification of the lens on a digicam.
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