Telecentric

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The Telecentric is an early telephoto lens made by Ross of London. It was designed by William Bielicke[1] and introduced around 1912[2] and was sold until 1922.[3] It was replaced with the Teleros soon after this, however.[1]

The Telecentric was made in lengths of 9, 11, 12, 13 and 17 inches, and in maximum apertures f/5.4 and f/6.8. Both of these have the same back focus, which is half the focal length. As can be seen in the illustration, the lens comprises five glass elements in two groups. In his lecture to the RPS, Clay[1] referred to the Telecentric in connection to the use of cemented groups of glasses of different refractive indices, saying that then fairly new types of glass available from Schott in Jena allowed improved correction of aberrations.

Clay mentioned the short back-focus of the Telecentric relative to its focal length, and the usefulness of this particularly on reflex cameras, which have limited bellows extension. Early Photography[4] shows a reflex camera of 1914, a quarter-plate N&S Patent Reflex, with an 11 inch f/6.8 Telecentric in two parts, which fitted to the front and back of a front shutter. The standard lens for the camera is mounted on a shortened thread, for quick changing between lenses, and the lens shade-flap can also be quickly exchanged for a bellows hood to serve the telephoto. When not in use, the telephoto lens was stored in a compartment in the camera. Before the Telecentric was available, this camera was supplied with a Busch Bis-Telar, from 1911.

Detectives at Scotland Yard bought an 11-inch Telecentric for surveillance of suffragettes in 1913.[5]

Telecentric has come to mean something very specific in optics[6]; telecentric lenses are used in machine vision and measuring applications, but are not of much interest in general photography. The Telecentric is not a telecentric lens.



Notes

  1. 1.0 1.1 1.2 Clay, Reginald S. "The photographic lens from the historical point of view", lecture given at a meeting of the Royal Photographic Society, 10 October 1922. Reported in the British Journal of Photography, Vol. 69, No. 3258 (13 October 1922), p.624.
  2. The Telecentric is described as new in the 1912 Ross catalogue at Ciné-Ressources, though not given the same prominence as the Xpres, which is given the front pages of the catalogue, perhaps suggesting the Telecentric was not that new. The 1914 catalogue, also at Ciné-Ressources, still describes the Telecentric as new.
  3. The lens still appears in the Ross catalogue The Choice of a Lens, around 1922 (the date December 1922 is given in a price-reduction announcement stuck on the front cover) at Camera Eccentric (archived).
  4. N&S Patent Reflex at Early Photography. 'N' was Arthur Newman (also, earlier, of Newman & Guardia), who designed the camera. 'S' was James Sinclair.
  5. Casciani, Dominic (2003) BBC News Online article, Spy pictures of suffragettes revealed, 3 October 2003.
  6. Optical measurement techniques with telecentric lenses at Schneider.
  7. Wilkinson, M, and C Glanfield. 2001. A Lens Collector's Vade Mecum, CD-rom Version 3F. Edited by A. N. Wright. Cornwall, UK: David Matthews Associates. Chapter 11, Pages 87-88. Cited in the article on Ross serial numbers.

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