光学望远镜的放大倍数是指被观测物体的张角，在经过望远镜的光学系统后被扩大多少倍。比如1000米外一个1米大小的物体，肉眼直接观测时，其张角约为0.001弧度；用放大倍数为10倍的望远镜观察该物体，其张角为0.01弧度，相当于肉眼从100米外观察该物体。

放大倍数的计算公式如下： 其中M表示放大倍数，F表示物镜的焦距，f表示目镜的焦距。

望远镜的放大倍数通常刻在镜身上，用倍数×物镜口径来表达。比如8×30，表示该望远镜放大倍数为8倍，物镜口径为30毫米。

Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in size, sometimes called "minification" or "de-magnification".