TELEPHOTOGRAPHY (Fr., Tilephotographie) The photography of distant objects by means of lenses giving high magnification compared with ordinary lenses used with the same exten sion of the camera. A telephoto lens is in fact, a long-focus lens requiring but a short camera extension ; it has not a definite or fixed focal length like other lenses, and it may be so adjusted as to give a sharp image at any extension of the camera, provided that the extension is not less than the focus of the positive lens which forms part of the telephoto lens.
The combination of lenses which forms the telephoto system consists of an ordinary photo graphic lens, preferably a rapid anastigmat, called the positive element ; and a negative lens, so arranged that its distance from the posi tive lens may be varied by means of a rack and pinion on the lens mount. The distance from the positive to the negative element and the distance from the negative lens to the sensi tive plate are variable, the former being deter mined by the latter, and on these distances depends the degree of magnification ; or, in other words, the proportionate size of the image compared with the size of the image yielded by the positive lens alone.
Unless the positive lens can work at f/8, focusing and the arrangement of the picture become very difficult. The value of the aperture of the positive lens is reduced by the negative element in direct proportion to the magnifica tion ; consequently, with a magnification of four times, f/8 becomes f/32.
For ordinary work, the focus of the negative lens should be about half that of the positive. If great magnification is required for special work, the focus of the negative lens should be still less. This short-focus negative lens has the disadvantage of reducing the covering power of the positive lens, and this is a serious but unavoidable objection to the telephoto lens system.
Although the focal length is lengthened, the area covered by the lens is reduced very con siderably ; but, with a given combination of lenses, the greater the magnification, the greater will be the covering power. It is very desirable that the focus of the negative element should be known, as it provides a definite basis for ascertaining the degree of magnification. Ernest Marriage gives the following simple rule for find ing the magnification when the focus of the negative lens is known : After focusing, divide the distance from the negative lens to the focus ing screen by the focus of the negative element, and add i to the result. Example.—A nega
tive lens of 4 in. focal length is used, and, when the image is sharply focused, the negative lens is 8 in. distant from the focusing screen. What is the degree of magnification ? 8 — = 2 ; 2 + r= 3 times.
4 Many telephoto lenses have a scale of magnifi cations engraved on the mount, so that, when the image is sharply focused, the degree of magnifica tion can be read off from the position of the indicator on the scale.
The working value of the stop can be ascer tained by multiplying the value of the aperture in the positive lens by the degree of magnification. Thu; with a magnification of three diameters, f/8 becomes f/24; and with a magnification of four times, f/r x.3 becomes about 1/45.
The size of the image increases with the exten sion of the camera, so that it is a very simple matter to secure any size that may be desired. If the image when first focused is too small, the extension of the camera should be increased, and then the subject re-focused by the rack and pinion on the lens mount. If it is too large, decrease the camera extension and re-focus.
Attention has been directed to the fact that the covering capacity of the telephoto lens is very small ; consequently, the use of the rising front becomes an impossibility, as the plate would show dark corners. Tilting the camera is neces sary for all subjects which would require the rising front when using an ordinary lens ; and when the tilting becomes excessive, as in photo graphing architectural details at a considerable height from the ground, it is not practicable to set the camera back vertical owing to the im possibility of securing good definition through out when the plate is at such an angle to the axis of the lens.
An objection to many telephotographs is the very slight perspective effect obtained. It is the extreme opposite to the exaggerated effect some times produced by a wide-angle lens, but at times it is quite as pronounced and as unnatural.
Two difficulties that become serious when working with a telephoto lens for distant sub jects are haziness in the atmosphere, which renders it impossible to secure clear detail, and wind, even a slight breeze introducing serious vibration with the long camera extension.