Teleconverter

Teleconverter (double)

Teleconverter (from ancient Greek τῆλε Tele , German , far ' and Latin convertere , turn' - in other words as "remote converter") are accessories for camera objective , the focal length zoom lens of the present. As a result, a smaller angle of view is captured and the image is enlarged accordingly with the same distance to the subject. A wide-angle attachment has the opposite effect .

Executions

Teleconverters are usually placed between the interchangeable lens and the camera . The information for autofocus , aperture control and so on can, depending on the version, be transferred electrically and / or mechanically by the converter between the lens and the camera. They are also available as converters for film and digital cameras with built-in lenses . Such an attachment is in principle a Galileo telescope .

Teleconverters are available in the form of 1.4-fold to 3-fold converters. The number mentioned indicates the ratio in which the focal length of the lens used increases, i.e. a double teleconverter doubles the focal length of the lens.

Several teleconverters can be added one after the other for even greater factors. At the end of 2010, hobbyists placed five or three double converters in front of an 800 mm telephoto lens and used them to create respectable lunar photographs.

Canon calls converters extenders .

Details

Olympus EC-20 - 2x teleconverter
1 - lens
2 - teleconverter
3 - camera

Front teleconverter

A double teleconverter increases the focal length of a 50 mm lens to 100 mm and that of a zoom lens with a focal length range from 24 to 85 mm to 48 to 170 mm. Since the close-up limit of the basic lens remains unchanged, a teleconverter also achieves a correspondingly larger maximum image scale . Teleconverters are therefore often an inexpensive, weight and space-saving alternative to another additional lens.

A disadvantage of a teleconverter that is placed between the lens and the camera is the reduced light intensity compared to the basic lens . Because the opening of the basic lens remains unchanged, the relative opening (opening by focal length = light intensity) is reduced by the factor of the increase in focal length. For example, a 500 mm / 1: 8 telephoto lens with a two-fold teleconverter has a focal length of 1000 mm, but only a light intensity of 1:16, two f-stops are lost, with a three-fold converter a correspondingly large number of three (3 , 2) f-stops, and one f-stop for a 1.4x converter. With an n -fold converter, the light intensity is reduced by the factor . Specified in f-stops is the loss of light intensity ${\ displaystyle n ^ {2}}$

{\ displaystyle \ log _ {2} n ^ {2} = 2 \, \ log _ {2} n = 2 \, {\ frac {\ log n} {\ log 2}} \ approx 6 {,} 644 \, \ log _ {10} n \ approx 2 {,} 885 \, \ ln n}

.

Teleconverters intended for universal use can be used sensibly with lenses for 35 mm cameras with focal lengths of 50 mm to 300 mm. With lenses with a longer focal length, with wide-angle lenses and so-called superzooms , the image quality with such converters suffers greatly. Some manufacturers therefore offer teleconverters that are optimized for a specific lens, a lens family or for a certain focal length range. Such individual converters impair the achievable image quality to a lesser extent.

disadvantage

Camera viewfinder with a 300 mm telephoto lens

Camera viewfinder with 300 mm telephoto lens and double teleconverter

The reduction in quality caused by a teleconverter is caused by:

The imaging errors of the basic lens are also enlarged by the converter.
The converter itself creates imaging errors which, in the worst case, increase the imaging errors of the lens.
The optical quality can be reduced by centering errors, i.e. if the optical axes of the basic objective and converter do not coincide.

If the converter is calculated for a certain basic lens, it can be achieved that the aberrations of the converter and lens compensate each other at least partially. In principle, the combination can even be better than the lens alone. However, the quality is also affected by manufacturing deviations of the basic lens. The effect of this is inevitably increased by the extension of the focal length. That is why the combination of converter and lens is usually worse than the basic lens in this case too.

A classic combination is the use of a double converter with a bright 2.8 / 300 telephoto lens, which results in a still easy-to-use 5.6 / 600 “super telephoto”.

It is possible to use several teleconverters at the same time, but the image quality and the light intensity are reduced accordingly.

Due to the reduction in the light intensity of the lens, it can happen that the autofocus sensor of the corresponding camera housing no longer receives enough light and the automatic distance setting no longer works correctly.

Purely optical teleconverters without electronic components cannot transmit any data between the lens and the camera housing, so that the corresponding information is not available. For example, an aperture number set on the lens or the status of the image stabilizer is not transmitted to the camera body, and the lens's autofocus motor cannot be activated in order to adjust the distance .

Web links

Commons : Teleconverters - collection of images, videos and audio files

Individual evidence

↑ lunar telescope with teleconverters (Engl.)

[1] unar telescope with teleconverters (Engl.)