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One zoom is one with focal distance variable: an additional ring makes it possible to modify the focal distance in a continuous way, which modifies at the same time the enlargement of the image but not the prospect for the catch of sight.
The zoom is technically an objective comprising several groups of lenses, some are mobile: what makes it possible to vary some between two extreme values.
The numerical relationship between longest and the shortest focal distance are called "amplitude" or "power" of the zoom.
For example, a zoom 35-105 mm is known as "zoom 3×".
The zoom thus replaces advantageously a series of objectives to fixed focal distance. The possibilities of the zooms increase quickly, the ranges from 28 to 70 mm (of great angle to normal) and from 70 to 200 mm being most usual. The maximum opening seldom exceeds f/3.5 for the first and f/4.5 for the seconds. But there are professional versions with f/2.8.
The zoom was invented in 1949 by French engineer Roger Cuvillier. The prime objectives zoom are the Side Cinor of the company Som Berthiot.
- Image:Zoom.GIF : for an animated example of zoom