Most SLR and DSLR cameras provide the option of changing the lens. This enables the use of lens that are best suited for the current photographic need, and allows the attachment of specialised 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, 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 DSLRs 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-3 and K-50. A Pentax K-mount lens from the early 70s can be used on the newest Pentax DSLR although it may not provide features that are included in newer lenses (e.g. autofocus). 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 (e.g. lack of autofocus 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). Adapters may not be available to bridge every combination of lens mount and camera mount.
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