The difference between a fixed focal length lens and a zoom lens is that with zoom lenses a photographer can vary the focal length within a pre-defined range. Zoom lenses make it much easier to produce a number of different perspectives or compositions, as it removes the requirement for lens changes. This is particularly important when photographing dynamic subject matter. When utilising a zoom lens a photographer will still need to change their position, as in fact the zoom lens just allows for improved flexibility.
A zoom lens is a combination of lens elements that provides the ability to vary its focal length. Zoom lenses are most often used with still, video and motion picture cameras. They can also be used with projectors, some binoculars, microscopes, telescopes, telescopic sights and other optical instruments. A true zoom lens is also known as a parfocal lens, as it maintains focus when its focal length changes. There are a wide range of zoom lenses, some are telephoto, some are wide-angle and there are others which cover a range from wide-angle to telephoto.
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. The term “superzoom “or “hyperzoom” is used to describe photographic zoom lenses with very large focal length factors, typically more than 4× and ranging up to 15× in SLR camera lenses and 26× in amateur digital cameras. This ratio can be as high as 100× in professional television cameras.
Many of today’s modern digital cameras have features which allow the cropping and enlarging of a captured image. These options are designed to simulate the effect of a longer focal length zoom lens. This is commonly known as digital zoom and produces an image of lower optical resolution than optical zoom.
An optical zoom is a true zoom, as the focal length changes and the zoom mechanism itself extends and retracts, so that an image is magnified by the lens itself and therefore an optical zoom produces the best quality images. A digital zoom however is not a true zoom but a simulated zoom that enlarges the central portion of an image in the camera. The actual length of the lens does not change, which is why digital zoom lenses produce images with reduced visual quality.
