For a more general discussion about lenses, see Photographic lens.
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.
As implied by the above, lenses are only interchangeable within the "mount system" for which they are built. It is not possible (at least not without an adapter) to mount, for example, a Pentax lens on a Sony camera body.
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 the lens focal length to its effective aperture diameter. A small f-number like 2.0 indicates a large aperture (more light passing 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 diaphram 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.
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 System camera, 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.
|Sensor size:||35 mm or full-frame||APS-H||APS-C||Foveon X3||Four Thirds|
|Crop factor:||1.0||1.3||1.5 or 1.6||1.7||2.0|
|Lens view|| Angle of view|
|Focal length (mm)|
|"Normal"||59 - 47||40 - 50||30 - 38||25 - 32||23 - 29||20 - 25|
|Very long telephoto||2||1200||923||774||706||600|
| Some cameras|
| 35 mm film|
Canon EOS-1Ds II
Canon EOS 5D
| Canon EOS-1D II N|
Canon EOS-1D III
| Canon EOS 400D|
Sony α 100
Sony α 300
Sony α 350
Sony α 700
Sony α 900
| Sigma SD9|
| Leica Digilux 3|
The focal length of a zoom lens is not fixed; instead it can be varied between a specified minimum and maximum value. Modern lens technology is such that the loss of image quality in zoom lenses (relative to non-zoom lenses) is minimal, and zoom lenses have become the standard lenses for SLRs and DSLRs. This is different from only 20 years ago when, due to image quality concerns, most professional photographers still relied primarily on standard non-zoom lenses.
Zoom lenses are often described by the ratio of their longest to shortest focal lengths. For example, a zoom lens with focal lengths ranging from 100 mm to 400 mm may be described as a 4:1 or "4×" zoom. Typical zoom lenses cover a 3.5× range, for example from 24 - 90 mm (standard zoom) or 60 - 200 mm (telephoto zoom). "Super-zoom" lenses with a range of 10× or even 14× are becoming more common, although the image quality does typically suffer a bit compared with the more traditional zooms.
The maximum aperture for a zoom lens may be same for all focal lengths, but it is more common that the maximum aperture is greater at the wide-angle end than at the telephoto end of the zoom range. For example, a 100 mm to 400 mm lens may have a maximum aperture of 4.0 at the 100 mm end but only 5.6 at the 400 mm end of the zoom range.
Standard non-zoom lenses are called prime lenses or simply "primes". Their advantage, in addition to typically giving a slightly better image quality, is that they are smaller, lighter and cheaper than a zoom lens of the same quality. A prime lens may also be "faster", i.e., have a larger maximum aperture (smaller f-number), so it can be used with less light (with the same shutter speed), and can provide less depth of field in situations where this is desirable.
Most users of SLR and DSLR cameras stick to using zoom lenses, while a few of the more adventurous amateurs and many professional photographers also invest in a few prime lenses. Special purpose lenses are, as the designation implies, for special purposes, and are not so common.
There are many different kinds of special purpose lenses, the most popular being fisheye lenses, which are extreme wide-angle lenses with an angle of view of up to 180 degrees or more, with very noticeable (and intended) distortion.
Some other kinds of special purpose lenses, such as perspective control lenses and soft-focus lenses, were more popular with film SLRs but are less popular for DSLRs because the same or similar results can be obtained with post-processing software.
For a more complete discussion of special purpose lenses see Special-purpose photographic lenses.
Almost all modern lenses for SLRs and DSLRs provide automatic focus. The autofocus sensor(s) and electronics are actually in the camera body, and this circuitry provides electrical power and signals to a motor inside the lens that adjusts the focus. (Some older autofocus systems are based on a motor in the camera body and using a mechanical connection to the focus mechanism in the lens.)
There are two different kinds of in-lens electronic focus drive motors currently in use, the traditional servo motor and the more modern "ultrasonic" drive systems. These ultrasonic drives go by different names according to the manufacturer, for example USM (Canon), AF-S/Silent Wave (Nikon), Super Sonicwave Motor/SSM (Sony), Supersonic Wave Drive (Olympus), Extra Silent Motor (Panasonic/Leica), Supersonic Drive Motor (Pentax), and HSM (Sigma). These ultrasonic focus drives typically provide faster focusing than the non-ultrasonic drives, as well as being practically silent and using less battery power.
Note that image stabilization can not reduce blur if the subject is moving, or if the camera is being panned in a continuous movement. Note also that mounting a lens with optical image stabilization on a camera with in-body image stabilization does not provide improved results; on the contrary, you must switch one of the two systems off.
This does not necessarily mean that one is limited to only mounting, for example, Pentax lenses on a Pentax camera body. There are independent optics companies that make lenses for the various otherwise proprietary mount systems, thus providing alternative sources for lenses that are often of equal quality and/or less expensive than the camera maker's own lenses. Another possibility is the use of adaptors that allow mounting a lens for one system on a camera with a different lens mount. However, the use of an adaptor usually results in reduced functionality, typically requiring the manual setting of aperture and focus, or perhaps not being able to use any aperture other than "wide open".
Canon introduced the EF lens mount in 1987 as part of the EOS system. It broke with the most common technique for implementing autofocus at that time by not having a mechanical connection to a motor in the camera body, having instead only electrical connections and requiring a motor to be part of each autofocus lens.
The EF-S lens mount is a newer subset of the EF standard, introduced in 2003. EF-S lenses can only be used on Canon digital cameras that use the APS-C sensor, for example the 400D (EOS Digital Rebel XTi) and the 40D. Note that while you can not mount an EF-S lens on a camera that uses the EF mount, the converse it not true; you can mount EF lenses on cameras designed for both the EF and the EF-S standards.
As noted above under focal length, Canon makes DSLRs with various sensor sizes, and all using the EF or EF-S lens mounts. This leads to the interesting phenomenon of the same EF lens providing different angles of view depending on which camera it is mounted on.
Third-party lenses compatible with Canon's EF and EF-S mounts are manufactured by Sigma, Tamron, and Tokina. The manufacturers of these lenses have reverse engineered the electronics of the EF lens mount. The use of these lenses is not supported by Canon. However, many users find these lenses to be cheaper, and sometimes superior alternatives to Canon lenses. A fairly complete list of EF and EF-S lenses can be found on the EFLens.com web site.
The Four Thirds System sensor size (17.3 mm x 13 mm) is the smallest currently being used in DSLR cameras. This leads to both advantages (theoretically smaller, lighter and cheaper lenses and camera bodies) and disadvantages (slightly lower image quality, especially in low-light situations).
There are currently over 35 lenses available for Four Thirds System cameras. A complete list can be found on Andrzej Wrotniak's web site.
In North America, Minolta began using the name 'Maxxum' for the SLR autofocus cameras, lenses and flashes while in Europe they were called 'Dynax', though they were otherwise identical in appearance and function - all of the equipment is 100% interchangeable regardless of which of the two names it carries.
All Minolta Maxxum/Dynax compatible lenses, whether built by Minolta or one of the aftermarket lens manufacturers, are focused externally by a shaft connecting the autofocus computer and motor inside the camera body that mechanically connects to the internal focusing gears inside of the lens body, unlike other SLR and DSLR systems (i.e. Canon and Nikon) where only the AF computer is inside the camera body and there is a digital interface connecting body to an electric motor and the focusing gears built into the lens body creating a "drive-by-wire" focusing system.
This unusual design has several benefits such as allowing for smaller and lighter lenses and also keeps the cost of lenses down because there are no internal focusing motors or digital interfaces built into the lens ... also, by keeping the autofocus motors inside the camera body and as far away from the lens glass as possible there is a vibration reduction benefit as well.
Most Nikon F-mount lenses cover the standard 36×24 mm area of 135 film, while "DX" designated lenses cover the 24×16 mm area of the Nikon DX format sensors, commonly referred to as APS-C format. "DX" lenses produce vignetting when used on film cameras or full frame digital cameras such as the new Nikon D3. However, the Nikon D3 has a DX-compatible mode that reduces the resolution from 12.2 megapixel to 5.1 megapixel and avoids vignetting.
Industrial F-mount lenses have varying, often small, film/sensor coverage. Older F-mount lenses designed for film cameras will work on modern SLR or DSLR cameras with some limitations, typically not providing autofocus or automatic aperture setting.
The Pentax K mount (or just "PK mount") was created by Pentax in 1975, and has been used by all Pentax 35 mm and digital SLRs since. The mount has been developed over the years, resulting in a large number of designations such as KF mount, KA mount, KAF mount, KAF2 mount and KA2 mount, plus a couple of more recent versions that are not completely backward-compatible and are thus referred to as "crippled" versions. ("Crippled" in this context does not imply any lack of modern functionality, just a lack of compatibility with past lenses.) For more information see the Pentax K mount article or Bojidar Dimitrov's web site.
A number of other manufacturers have produced K-mount lenses, and several other manufacturers have made K-mount cameras. In 2005 Pentax and Samsung entered into a cooperation resulting in the Samsung GX line of DSLRs, based largely on Pentax technology including the Pentax K mount.
The Sigma DSLR cameras that use the SA mount are the Sigma SD9, Sigma SD10 and Sigma SD14. These cameras are noteworthy for their use of the Foveon X3 sensor, an image sensor that works on quite different principles from the sensors used in all other digital cameras.
Sony acquired Konica Minolta's camera technologies in 2006, and chose the "α" (alpha) brand name, already in use by Minolta in Asia, for their new "Sony α" digital SLR system. The Minolta AF lens mount was retained from the old cameras and is now officially known as the "Sony α mount system".
Sony has produced several new lenses for the Sony α mount, and the current list of Minolta and Sony α mount lenses has over 60 entries. Some of the newest α mount lenses are designated "DT" for Digital Technology; these are for digital cameras with APS-C sensors, and will result in vignetting if used on a film SLR.
Canon, Olympus, and other manufacturers have changed lens mounts. Much older Canon film camers used the FD lens mount, which was discontinued in 1987 in favor of the EF lens mount. Olympus discontinued the MD lens mount for the OM series cameras in favor of the Four Thirds System lens mount.