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

- Short Telephotos Lens
- Medium Telephotos Lens
- Long Telephotos Lens
- Mirror Lenses and Telescopes
- Tele Extender
- Characteristics of Tele Lenses

- Tips on how to handle long lenses

 

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