The Tele Extender – a lightweight telephoto

The tele-extender (or tele-converter) is a small lens system in itself, which, however, must be used in conjunction with a prime lens in order to form an image. It increases focal length when fitted between any lens and the camera body, depending upon the strength of the particular extender. The most popular size is 2 times, which doubles the magnifying power of a prime lens, although 1.5 times and 3 times extenders are also quite useful.

Early versions of this accessory were not always satisfactory. They were meant to be used with any lens and camera of any make that had mounting flanges of the proper size and type. Naturally there had to be compromises in the design of an extender with such broad applications. There was an inevitable loss of sharpness in the photographic image for most users, sometimes more than even a casual picture taker could accept. However, over the years quality has constantly improved, until today extenders deliver amazingly good results.

Another development has further increased the image quality of some extenders; they have been matched to specific lenses or lens groups. Instead of trying for a general purpose design that will largely satisfy the optical requirements of several quite different lenses, many manufacturers are now using computers to pair an extender with a specific lens for the utmost in compatibility. With this approach an extender becomes part of the lens itself rather than a mere add on. Sharpness and ease of use both increase. While general purpose tele extenders should be closed down at least one full f-stop for satisfactory results, the new matched extenders are good even when used wide open.
 

The above advantage is quite important, because there is a loss of lens speed with any extender. A 2x extender will double the focal length of a prime lens, but it will also steal two f-stops. A 135mm telephoto lens, for example, can become equivalents of 270mm with a 2x extender in place. If the lighting conditions call for f/8 with a prime lens alone, they will now require two additional f-stop (f/4) with the accessory. However, if the maximum aperture of your 135mm lens is only f/4 and you have to step down to get a sharper image, it means only a slower shutter speed will produce the correct exposure. With 27mm of magnification that could mean un-sharp picture just from camera movement. Therefore, it is easy to see why being able to use tele extender wide open is so convenient. Because the speed loss accompanying tele extender use, some manufacturers provide 1.5x extenders. They don’t magnify as much, but they don’t require as much exposure increase either.
 

Extender can be used in combination of further augment lens power, but the practice is not advisable. Using two extenders at the same time will only add to any sharpness problems, and exposures loss can be extreme. A firm tripod and a slow shutter speed may be the only answer in such a situation. A through the lens automatic exposure camera will do all the exposure calculations for you, but if you are using a manual camera, some on the spot math will be needed.
 

Tele extenders are available in manual versions, but the trend is to automatic functioning so that none of the auto features of the prime lens are lost. Some manual units allow removal of glass elements so that the barrel can serve as an ordinary extension tube.
 

While tele extenders cost lens speed and occasionally some sharpness, they do provide real advantages. They are compact and lightweight compared with prime lenses of the size they replace, and they are certainly less expensive. The size/weight advantage is especially important to a traveler attempting to get the most versatility from the least amount of equipment. With 50mm and 200mm prime lenses plus a 2x extender, two extra focal lengths (100mm and 400mm) can be had for no more cost or weight than that of the extender alone. Although a prime telephotos lens is usually more desirable than a lens/extender combination, there are times when a tele extender can make your photographic work much easier.

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More about telephotos lenses

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