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7. Special Lenses (continued)
7.2 Zoom Lens
Everybody knows zoom lenses: In video and still cameras they achieve continuously variable magnification and even telescopes and microscopes look old-fashioned without this zoom feature.

In industrial optics, we could also enjoy zoom advantages:
With conventional lenses, the process of iteratively adjusting object distance and image distance for achieving correct scale and definition is very time consuming. (Object distance and image distance are explained in paragraph 1, figure 1.)

With a zoom lens, this would be greatly simplified: Once you have optimized definition by adjusting one distance, you can independently optimize scale by adjusting the focal length.

But zoom is not easy to make. A multi-element lens system is needed in which some elements must perform sophisticated movements when focal length is adjusted. You see: focal length shall be adjustable without changing image distance at a given object distance.

So for a given imaging specification, zoom lenses always contain more elements and more mechanics and are more expensive and heavier than a conventional lens. Or they exhibit less resolution, more vignetting, more distortion ... than the conventional lens.

There is a "little brother" of the zoom lens; people call this brother "vario lens" or "varifocal lens". Here, you can adjust for various focal lengths like with the "true" zoom lens. But after every focal length adjustment, you must re-adjust the image distance.
Fixed distance between lens system housing and image plane is the domain of true "zooms". And their great advantage.

Conclusion in short:

Zoom advantage: Independent adjustment of scale and definition.
(Motorized) field angle adjustment in the operating optical system.

Zoom drawback: At same imaging performance, zoom lenses have more size, weight, and price than conventional lenses.

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Last modified Nov.29th, 2002 00:19