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 the late 1980s when, due to image quality concerns, most professional photographers still relied primarily on standard non-zoom lenses. However, zoom lenses still typically have a lower maximum aperture than fixed-focal ("prime") lenses for the same weight and cost, especially for shorter focal lengths.
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 the same (constant) 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 f/4.0 at the 100 mm end but will diminish to only f/5.6 at the 400 mm end of the zoom range. Zoom lenses with constant maximum apertures (such as f/2.8 for a 24-70mm lens) are usually reserved for lenses with higher build quality and are thus more expensive than those with variable maximum apertures.
Prime lenses
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.
Macro lenses
Macro lenses are designed for extreme closeup work. Such lenses are popular for nature shooting such as small flowers, as well as for many technical applications. As most of these lenses can also focus to infinity and tend to be quite sharp, many are used as general-purpose optics.
Special purpose lenses
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 (sometimes 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
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results can be obtained with post-processing software.
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results can be obtained with post-processing software.
For a more complete discussion of special purpose lenses see Special-purpose photographic lenses.
Automatic focus
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" drivesystems. 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 Hypersonic Motor/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.
Optical image stabilization
Image stabilization is a technique used to reduce image blur caused by the camera not being held steady. There are two kinds of image stabilization used in SLR and DSLR cameras and their lenses:
- In-body image stabilization is implemented by moving the image sensor in an attempt to counteract the sensed motion of the camera. The advantage of this technique is that it works for all lenses mounted on the camera, at least if the camera electronics are aware of the lens' focal length. This is most commonly done automatically, but some cameras (such as all Olympus bodies with IS) allow the user to input the focal length manually for use with lenses with no electronic coupling. In-body image stabilization is used in modern Olympus, Sony, and Pentax cameras.
- In-lens image stabilization is implemented in the lens itself, and moves the lens elements in an attempt to counteract the sensed motion of the camera. The inherent advantage of this kind of image stabilization is that it steadies the viewfinder image, allowing for more accurate framing and autofocus. The disadvantage is that you have to pay the extra cost for every lens you buy for which you want image stabilization.Panasonic, Canon, and Nikon use lens-based image stabilization. Some third-party lenses from Sigma and Tamron also have lens-based IS systems.
The effectiveness of image stabilization systems varies somewhat from implementation to implementation, but there seems to be no inherent superiority to either lens-based or sensor-based systems as far as the actual improvement in captured images.
Image stabilization systems can degrade image quality if the photographer is intentionally panning (as the system tries to negate the panning motion), or if the camera is mounted on a very sturdy tripod (the system drifts around slowly due to spurious measurements over the course of a long exposure). Some more recent IS systems can automatically detect these situations and disable the IS along the panning axis, or disable it completely if the camera is on a tripod.
Mounting a lens with optical image stabilization on a camera with in-body image stabilization does not provide improved results, since the combined effect of both systems will "overcorrect". Users of image-stabilized lenses on bodies with sensor-shift IS should determine which system offers superior performance and turn the other off.
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