Zoom lenses (those in which focal length and, hence, the field of view can be adjusted without affecting focus) are fitted almost universally to non-SLR digital cameras. Not only are these zoom lenses convenient for the photographer - they provide a choice of subject framings and magnifications all from the same shooting position - their optical form also allows camera designers to build more efficient autofocusing systems into them. The benefit of this from the photographer's point of view is a less-bulky lens system that requires less battery power to operate.

Most zoom lenses on digital cameras start at quite a moderate wide-angle focal length of a 35efl of approximately 35mm. More extreme focal lengths are difficult to achieve because of the optical characteristics of the small photosensors used for image capture, which tend to cause a darkening at the periphery of wide-angle images. When wider angles of view are required) accessory lenses can be screwed into the front of the digital zoom to extend its range, though image distortion is likely as a result.

At the other end of the zoom range, however, lenses can reach high image magnifications - a 35efl of 350mm in one model - although focal lengths of around 120mm are more usual. Be aware that along with magnifying a small part of the field of view to fill the frame, long focal lengths also magnify the effects of camera shake on the image. Some cameras and lenses, notably those from Olympus and Canon, respectively, offer technologies that significantly reduce the effect of camera shake.

The greatest range of lens focal lengths are available only with digital SLR cameras. With these cameras, zoom lenses can extend from ultra wide-angle focal lengths of, say, 17mm to a moderate 35mm, or from a modest 100mm focal length to a 560mm super-telephoto.

The most tempting zooms are those covering a range from wide-angle through to telephoto - 28-300mm, for example, or 35-350mm. The best of modern designs can deliver excellent image quality over these ranges, but a high financial price needs to be paid. You pay in another sense, too: maximum apertures are often limited, making them suitable for use in bright light only, and the lenses are large and heavy.

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