According to the zoom range, camera lenses can be divided into wide-angle lenses, standard lenses, and telephoto lenses. Focal length is a basic characteristic of the zoom lens. It can determine the magnification ratio of the image and the horizontal field of view captured by the lens. Focal length is determined by measuring the distance from the optical center of the lens to the point where the light converges behind the lens to produce a clear image. For television cameras, this point is the target surface of the CCD. The shorter the focal length of the zoom lens, the wider the horizontal field of view. Therefore, the image will be smaller. The horizontal field of view narrows with the increase of the focal length, and the captured object becomes larger accordingly.
The biggest characteristic, or value, of zoom lenses is the ability to change the focal length at the photographer's will. Unlike fixed focal length lenses, zoom lenses do not rely on quick lens changes to change the focal length. Instead, by pushing or rotating the zoom ring of the lens, the focal length can be changed continuously within the zoom range. This provides conditions for diversified composition.
Any intermediate focal length of a zoom lens has the same function as a fixed focal length lens with the same focal length. However, zoom lenses do not limit the photographer to using any particular focal length, making them much more convenient and flexible to use. It eliminates the trouble of carrying and changing multiple lenses of different focal lengths when shooting outdoors. Even just before taking a picture, photographers can crop the image by changing the focal length of the lens to achieve better composition.
The zooming speed of a zoom lens is incomparable to that achieved by changing the lens of a fixed focal length lens. Some zoom lenses also use electric zoom mode, which not only saves effort and is convenient, but more importantly, achieves uniform speed zooming. This is very helpful for photographers to cut the image by subtle changes in focal length and determine the composition. By zooming at the moment the camera shutter opens, some zoom lenses can also achieve an "explosion effect." Some cameras also rely on automatic control of the zoom lens's focal length change to achieve automatic composition functions.