The construction of telephoto combinations is altogether dis tinct from that of ordinary photographic lenses. It is based on the fact that, if a negative lens is placed behind a positive lens at a separation not less than the difference between their focal lengths (which constitutes the Galilean telescopic condition), and not more than the focal length of the positive element, a photo graphic combination is formed, the focal length of which increases as the separation is decreased. The "back focus," or distance from the back lens of the combination to the plate, is not, however, correspondingly increased, being dependent wholly on the focal length of the negative element, a very important consideration as it enables considerable magnification of the image transmitted by an ordinary lens to be obtained with the same or less camera extension. In 1907 Capt. Owen Wheeler introduced a system of combining tele-negatives to produce compounds of extremely short focal lengths, with which magnification up to X 3o is possi ble with a camera extension of only about 13 inches. A good feature of this system is that by making the rear lens of the com bination one of relatively large diameter the covering power is greatly increased. Advantage was taken of this fact in a special complete tele-lens freely used by the Royal Engineers in the World War, which had a focal length of 3o in. and covered a 5 X 4 plate with a camera extension of 6 in. only.
Originally telephoto lenses were "variable power" ones—i.e., the separation of the elements could be altered at will to pro duce different magnifications with the same or with different tele negatives. Also an ordinary photographic lens was generally used as the positive element. These arrangements were defective owing, firstly, to the disturbance of the corrections of the positive by a shifting negative element, and, secondly, to the comparative slow ness of the combination due to the fact that the aperture of any telephoto lens is the aperture of the positive multiplied by the magnification. Thus, in the case of a variable power combination
in which, in order to obtain good definition, the positive is stopped to, say, f/ii, the aperture at only 5 magnifications would be f/55.
Another drawback in the case of variable power combinations is their liability to internal reflections owing to the relatively long separation of the two elements. This is partially overcome by lining the lens tube with some non-reflecting substance or ma terial, but a more effective plan is to fit a hood on the lens to cut out the extraneous light— a method often adopted with advantage in the case of ordinary objectives.
Variable power telephoto lenses are still used by specialists and extraordinary results are attainable with them. But in mod ern practice single-focus combinations are much more common.
In these the positive element is a cemented single lens of ex tremely wide aperture, the defects of which are corrected in the negative element. The earliest examples of this form were the f/9 Busch Bis-Telar (1905) and the 1/14 Zeiss "Magnar" (1906), both since improved, but these have been put into the shade by recent productions of Ross, Dallmeyer, Taylor, Taylor and Hob son, Zeiss and others. The principles and construction of the Telephoto Lens are discussed at length by H. W. Lee in the Photographic Journal for 1925, p. 392.
See Proc. Physical Soc. of London for 1935, Vol. 47, P. 502, Taylor and Lee for full summary of developments.