Labels: LensesOther specialized lenses include the extreme telephotos mentioned previously (any lens longer than 500mm is a fairly specialized affair for sports and wildlife photography, and likely comes with a premium price tag, as well) and mirror lenses (which offer a more affordable, but less optically pure extreme telephoto up to 1000mm and more).
Sometimes you'll hear photographers refer to the speed of a lens, calling one "faster" than another. This is actually a reference to the maximum aperture of a lens, a measurement of how wide it can open up. The larger that aperture (the variable opening in the lens through which light passes), the more light can be let into the camera and, therefore, the faster the shutter speed that can be used.
Smaller ƒ-stops (like ƒ/1.4 or ƒ/2) indicate larger openings and faster lenses. These are desirable for many reasons, but especially for photographers who need to work at high shutter speeds (like sports and wildlife photographers) and in low light (such as photojournalists and documentary photographers). Another reason a photographer may prefer a fast lens is to produce a very shallow depth of field because the larger the aperture, the shallower the depth of field.
Prime lenses tend to be faster than zooms because they're optically simpler and, therefore, more easily capable of transmitting light efficiently. Some zooms actually incorporate a variable maximum aperture (like ƒ/4-5.6) that changes based on the focal length dialed in. This creates smaller and lighter zooms that cover a broader range of focal lengths.
The other "speed" of a lens is in the form of autofocusing speed. Many lenses have fast "supersonic" focusing motors built in to allow them to focus quickly, while others rely on a focus motor built into the camera. In any case, terms like "hypersonic" and "silent wave" denote variations on fast, quiet focusing mechanisms—which is what you want. Some lenses don't autofocus at all, though they're increasingly rare these days. This usually is found in specialty lenses. Fast focusing is important if you shoot fast subjects, but otherwise this feature may feel like a luxury.
Also called vibration reduction or optical stabilization, these lenses incorporate electromagnets and gyroscopic sensors to aid photographers who handhold their cameras. This is especially valuable when working with long lenses (as camera shake is amplified in pictures made with telephotos) and in low light or with slower maximum apertures.
Today's advanced stabilization technologies easily add two stops of handholding power. In layman's terms, that means if you're using a 100mm lens at 1/125 sec. at ƒ/2.8, to ensure a sharp image, you could effectively slow the shutter speed two more stops (to 1/30 sec.) and still produce a steady, sharp shot. Some lenses even claim up to four stops of stabilization, which can be invaluable for long lenses and low-light photography. (With a camera on a tripod, you must switch off the stabilization motors because they're counterproductive to the already steady tripod.)
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