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Excerpt from Field Guide to Geometrical Optics
A zoom lens is a variable focal length objective with a fixed image plane. The simplest example consists of two lens elements or groups (powers θ1 and θ2) where both the system focal length f and BFD vary with element spacing t.
As the separation approaches the sum of the element focal lengths (f1 + f2), the system becomes afocal (f→∞). The zoom range of the analogous telephoto zoom is limited by its BFD as the rear element can run into the image plane when the element separation approaches.
A mechanical cam provides the complicated lens motions required for these mechanically compensated zoom lenses. Zoom lenses often use multiple groups of moving elements. A common three group configuration uses a fixed front element and moving second and third groups.