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The camera uses an infrared beam or the subject's contrast to measure the camera to subject distance and focuses the lens automatically
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Called 'extender', 'lens extender' or 'telephoto extender' by some manufacturers. (Not to be confused with an 'extension tube'!). A teleconverter is an accessory that fits between the camera lens and body to increase the focal length of a lens by 1.4x, 1.7x, 2x or 3x. When coupled with a 200mm lens, for instance, teleconverters would give these results: a 1.4x teleconverter gives an effective focal length of 280mm, while a 1.7x teleconverter increases this to 340mm, a 2x teleconverter to 400mm and a 3x to no less than 600mm. Teleconverters are compatible only with selected lenses, so always check with the manufacturer or retailer before buying. Although they will work with some zoom lenses, they're best used with (fast) prime lenses, since there usually is a drop in quality, and many prime lenses give a higher level of quality to start with. Besides changing the effective focal length, the effective aperture of the attached lens is increased by one or more stops as well. Autofocusing usually does not operate if the effective maximum aperture is greater than f/5.6 with the lenses/cameras from most manufacturers (in some cases greater than f/8, like on the Canon EOS 1V and EOS 3 cameras). And not all teleconverters support autofocus in the first place!
The action of adjusting the lens so that the subject appears sharp on the film or CCD. Most cameras now have automatic focus (AF), some have manual focus override (MF) and some have a fixed focus (F) lens that ensures things from about 1.5 meters to infinity are relatively sharp. A few cameras have a power focus (PF), which is a manual method but using a motorised focusing ring. Buying advice: Autofocus is essential if you want to use the camera effortlessly, but occasionally the system is fooled, so make sure the camera has a focus lock or infinity setting or, better still, full manual control. Also check what the closest range is. Some only go down to about one meter which is useless if you want to fill the frame with a small object such as a flower or piece of jewellery.