Mirror lenses and telescopes – Seeing farthest

When focal lengths become so great that even a compact telephoto design is far too heavy and bulky for practical use, the mirror lens comes into its own. The mirror lens was developed by astronomers to obtain as much magnification as possible while keeping total physical length to a minimum. It is the type of lens used in the world’s largest telescope such as “Hale” in California’s Mount Palomar. The name is derived from the fact that two curved mirrors replace most of the usual transparent glass elements. The result is a lens containing both reflecting (mirror) and refracting (glass) components.

When light enters a mirror lens, it does not travel straight through as with a conventional design. Instead it doubles back on itself twice, bouncing off each of the internal mirrors in turn. By folding and compressing light rays in this manner, photographic lens designers fit a long focal length into a relatively short lens mount.
 

A further advantage is that curved mirrors weight less than a combination of glass elements. As a consequence a mirror lens is shorter, lighter and easier to handle than a conventional lens of the same focal length. In addition, a well made mirror lens is less prone to certain types of aberrations than most refractive (non-mirror) lenses.
 

All these desirable characteristics would seem to make mirror lenses the obvious favorites in long focal lengths, but such is not always the case. The mirror design introduces some peculiarities that many photographers fid bothersome.
 

First, because of optical restrictions, a mirror lens contain no iris diaphragm and must be used at its maximum aperture at all times. This aperture is generally in the f/8 to f/11 range, and although small by normal lens standards, it is often too large for sufficient depth of field with a long focal length lens. At the same time it doesn’t admit enough light to allow easy focusing or the fast shutter speeds needed to overcome highly magnified camera vibration. Furthermore, with no diaphragm present only shutter speeds, ISO setting and filters can be used to control exposure.
 

Another peculiarity of the mirror lens is the little “doughnuts” shapes are lens creates of out of focus points in a picture. These “doughnuts” are simply reflections of one of the circular mirrors. While deemed objectionable by many photographers, the tiny circles are though by others to add interest. Individual attitude often depends upon subject matter as well as personal taste.
 

Although mirror lenses are obviously most helpful in very long focal lengths, few photographers need such enormous reach. A more active market exists in the medium range, where the typical mirror lens has a focal length of 500mm and an aperture of f/8. Shorter focal lengths with larger apertures do exist, but they are rare.
 

Besides using compact mirror lenses or conventional telephoto lenses when taking pictures of distant subjects, photographers often adapt ordinary telescopes, binoculars or monocular to their cameras. These devices, designed primarily for visual use, are not as well corrected for photography as lenses made expressly for that purpose. However, they are generally much less expensive and serve the occasional user satisfactorily.
 

One characteristic common to all long focal length lenses, no matter what their design is the need to focus carefully and precisely. There is virtually no depth of field to mask careless focusing with any of them, and accuracy becomes more and more critical as focal length increases.

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More about telephotos lenses

- Short Telephotos Lens
- Medium Telephotos Lens
- Long Telephotos Lens
- Tele Extender
- Characteristics of Tele Lenses
- Tips on how to handle long lenses