Seeing Far (Tele lens)
Lenses that magnify a subject right on the sensor are known as either
long focus or telephoto lenses. The two terms are sometimes used
interchangeably, but there is a technical difference. A long focus lens
has a physical length that closely matches its focal length while the
overall length of a comparable telephoto lens may be only about half as
much. Telephoto lenses, therefore, have less weight and bulk than
conventional long focus lenses of the same focal length, making them
easier to carry and use.
The clever arrangement of lens components which shortens a telephoto
lens also creates a minor disadvantage. Aberrations cannot be corrected
as well as in a conventional lens of the same focal length. As a
consequence the maximum aperture must be kept smaller if equal image
quality is to be maintained. In view of this drawback there is little
advantage in using the telephoto design with lenses up to two or three
is little advantage in using the telephoto design with lenses up to two
or three times the normal focal length. Conventional long focus lenses
in this range are still small and compact enough to be practical.
However when focal lengths and magnifications become greater, it is
increasingly important to trim weight and bulk. Therefore, it is in
these longer focal lengths that telephoto lenses become not only
practical, but necessary.
No matter whether a lens is long focus or telephoto, it is really only
the focal length which is significant. The focal length has a direct
bearing on magnification and field of view, whereas exact knowledge of
optical design means little. Most manufacturers have stopped
differentiating between the two types of long lenses. As a result, the
term telephoto is now used exclusively by many photographers whenever a
longer than normal focal length is mentioned. Although it is not really
accurate to call every long lens a telephoto lens, it does simplify
communication. For this reason the same convention is adopted in this
book.
Telephoto lenses range in size from modest optics just a bit longer than
a normal lens to giants resembling astronomical telescopes. We will sort
telephoto lenses into groups and separately describe each one.
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More about telephotos lenses
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Short Telephotos Lens
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Medium Telephotos Lens
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Long Telephotos Lens
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Mirror Lenses and Telescopes
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Tele Extender
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Characteristics of Tele Lenses
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Tips on how to handle long lenses
Back to Camera Lens Features
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