Compact zoom lenses

Because early photographic zoom lenses replaced either fixed focal length lenses or afocal attachments placed in front of the objective lens, it is not surprising that they contained a fixed rear portion that resembled a conventional photographic objective •and was referred to as a "prime" lens. The front zooming portion was usually afocal and, while normally only two groups are necessary to zoom and maintain focus, the need for a separate focusing group dictated the use of a third front group. Thus, the four-group zoom lens conceived.

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