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5.1 Types of Zoom Lenses There are two types of zoom lenses, optically compensated and mechanically compensated. 5.1.1 Optically compensated zoom lens In an optically compensated zoom lens, two or more alternate lenses are linked and move together with respect to the lenses between them (refer to Fig. 5.1). This arrangement simplifies the mechanical construction and helps to maintain good control of boresight and alignment. The image motion produced by this type of system is a cubic curve, with the image in focus at a specific number of positions during zoom. The maximum number of positions where the longitudinal image motion is zero is equal to the number of moving air spaces. If the object is at infinity and the first lens group moves during zoom, the space between this lens group and the object is not considered a variable air space. A. D. Clark has developed one form of the calculations used to evaluate the image shift movements in an optically compensated zoom lens. Figure 5.2 represents a general three-lens zoom unit, with the zoom elements in their midzoom positions2 and also displaced from the midpoint.
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