Mirror lenses and telescopes – Seeing farthest
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
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.