Teleconverters are accessories for camera lenses that increase the focal length of the existing lens. Thus, a smaller angle of view is captured and magnifies the image at the same distance to the subject accordingly. The opposite effect is a wide angle attachment.
Teleconverters are typically mounted between interchangeable lens camera. The information for autofocus, aperture control, etc., depending on the version, are transmitted electrically and / or mechanically by the converter between the lens and camera. For film and digital cameras with built lenses they are also available as a front converter. Such intent is a Galilean telescope in principle.
Teleconverters come in the form of 1.4 -fold to 3 -fold converters. The above figure indicates the ratio in which increases the focal length of the lens used, ie a 2 -fold teleconverter doubles the focal length of the lens.
For even larger factors also several teleconverter can add one after the other. Hobbyists have set the end of 2010 five and three 2-way converter before a 800 - mm telephoto lens and thus created respectable moon photographs.
Canon refers converter as an extender.
A two -way tele-converter increases the focal length of a 50mm lens on 100 mm and a zoom lens with a focal range of 24 to 85 mm to 48 up to 170 mm. Since the minimum focusing distance of the prime lens remains unchanged, this is achieved at the same time a correspondingly larger maximum magnification ratio with a teleconverter. Teleconverters are so often a low-cost, weight and space-saving alternative to a further additional lens.
A disadvantage of a teleconverter, which is placed between the lens and camera, is the reduced light intensity compared to the base lens. Because the opening of the base lens remains unchanged, the relative aperture is reduced (limited by focal length = light intensity ) by a factor of focal length increase. For example, a 500 mm / 1:8 telephoto lens with a 2x tele-converter does have a focal length of 1000 mm, but only a light intensity of 1:16, there go two stops lost, according to abundant on a 3 -way converter three (3.2 ) f-stops, and a 1.4 -fold converter one f-stop. In an n -way converter, the light intensity is reduced by a factor. In f-stops indicated the intensity loss is
For universal use imaginary teleconverter are in lenses for film cameras with focal lengths from 50 mm to 300 mm useful usable. For lenses with a longer focal length, wide-angle lenses and so-called super zoom, the image quality suffers greatly with such converters. Some manufacturers therefore offer Tele-Converter, which are optimized for a particular objective, an objective family or for a certain focal length range. Such individual converters degrade the achievable image quality to a lesser extent.
The reduction in quality by a teleconverter arises, for example by:
- The aberrations of the basic lens are also magnified by the converter
- Through the converter arise even aberrations that increase in the worst case, the aberrations of the lens.
- The optical quality can be reduced by centering, ie when the optical axes of the basic lens and converters do not coincide.
If the converter for a particular basic lens is expected, one can achieve that the aberrations of the lens converter and compensate at least partially mutually exclusive. In principle, the combination may be even better than the lens alone. However, the quality is also affected by manufacturing variations of the basic lens. Their impact is also magnified inevitably by the focal length. Therefore, the combination of a converter and the lens is mostly worse than the base lens in this case.
A classic combination is to use a 2-way converter with a bright 2,8 / 300 telephoto lens, which is a more easily manageable 5.6 / 600 - results in " super-telephoto ".
Theoretically and practically did the simultaneous use of several teleconverter is possible, but the picture quality and the intensity decrease accordingly.
Auto Focus: Because the aperture of the lens is reduced, may time some Kamera-/Objektivkombinationen the autofocus. How lenses transmit some teleconverter their properties to the camera. Here taping some contacts in the bayonet can later reactivate the auto focus.