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变焦距镜头的焦距是由组成该物镜的各个透镜组的焦距和它们之间的间隔决定的。组成镜头的各透镜组的焦距在镜头装成后是不能改变的。故变焦距镜头只是用改变各镜组之间的间隔的办法,以达到镜头焦距改变的目的。为了改变镜组间的间隔,通常是靠一个带有变倍曲线槽和补偿曲线槽的园柱凸轮来实现的。当凸轮转动时,变倍和补偿曲线槽控制着变倍镜组和补偿镜组作相对应的轴向移动,使镜头焦距在一定范围内作连续变化。
The focal length of the zoom lens is determined by the focal length of each lens group constituting the objective lens and the interval between them. The focal length of each lens group that makes up the lens can not be changed after the lens is installed. Therefore, the zoom lens is only used to change the interval between the various groups of ways to achieve the purpose of changing the lens focal length. In order to change the interval between the mirror group, it is usually achieved by a cylinder cam with a variable-fold curve groove and a compensation curve groove. When the cam is rotated, the zooming and compensating curve groove controls the corresponding axial movement of the zoom lens group and the compensation lens group so that the lens focal length changes continuously within a certain range.