Modern Lens Design, 2nd Edition

Chapter 20Zoom Lenses

20.1 Zoom Lenses

Other than a single component working near unit magnification, the simplest zoom lens consists of two components with a variable airspace between them to change the focal length, plus, of course, a provision to shift the entire system to keep it in focus. To get the maximum change of focal length, one component is usually made positive and the other negative. Thus the two possible arrangements are similar to either a telephoto or a retrofocus, depending on whether the positive or negative component is on the object side of the lens. The telephoto arrangement is obviously more suited to longer focal lengths, and the retrofocus arrangement to shorter focal lengths and wider angular coverage. Figures 20.1 ...