|Helios x3 and x2 teleconverters|
image by Brian Eager (Image rights)
A teleconverter is a lens of low negative power, fitted in a lens tube and mounted behind any camera lens (i.e. between it and the camera), serving to increase its effective focal length. It is a cheap way to achieve a long focal length, though the picture is likely to be inferior to what could be achieved with a single lens of the same focal length, especially away from the centre of the picture.
Teleconverters are commonly supplied in powers 1.4x, 2x and 3x (they multiply the focal length by those factors). It follows that any given aperture setting on the main lens is (respectively) one-and-a-half, two, or three stops smaller when used with the converter. For example, a 200 mm f/4 lens, mounted with a 2x teleconverter, becomes a 400 mm f/8.
Teleconverters have been used since the late 19th century. Indeed, early 'telephoto lenses' were kits comprising the main lens, negative lens (a teleconverter), and a tube of variable length, allowing a range of focal lengths. Dallmeyer made the Adon, an assembly like this, in 1901, with a rack-and-pinion mechanism for varying the separation of the positive and negative parts, each a cemented doublet. Lan-Davis gives a table of the bellows extension, magnification and aperture for the Adon for focal lengths between 12 and 49½ inches. The Adon could be used either alone or mounted at the front of the camera's existing lens.
Soon, however, makers began to supply complete lenses, effectively a lens with a teleconverter built in permanently, in which the positive and negative parts were designed to complement each other, since it was found that lens aberrations were often magnified by the use of a converter not designed for use with a particular lens. These telephoto lenses gave better quality images, at the expense of the ability to vary the focal length. Examples include the Busch Bis-Telar of 1905, and the Ross Telecentric of 1912.
- Dallmeyer, T.R. (1899) Telephotography: an elementary treatise on the construction and application of the telephotographic lens, William Heinemann, London. Archived in various formats at the Internet Archive.
- Lan-Davis, C.F. (1912) Telephotography, George Routledge & Sons Ltd, London. Archived at the Internet Archive.
- Early Photography shows a number of early telephoto lenses and telephoto adapters, including one by Dallmeyer.
- Adon at Early Photography.