Some special forms of lenses will now be considered. The Petzval portrait lens, Fig. 25, has been in use for sixty years, not only as a portrait lens, but also as a projection lens. It works at a large aperture; but, is of short focus and has a very small field of view. If it is stopped down, flare is very likely to be caused. Several improvements on the form shown have been made. Perhaps the most notable is that in which the elements of the back component are mounted so that they can be separated at will. Any desired amount of diffusion of focus can be caused in this way. Another lens of long standing is the so-called Orthoscopic lens of Voigtlander, produced in 1857. This lens was of fairly long focal length, and yet did not require a corresponding amount of camera extension. The point P' in Fig. 13 was some distance in front of the lens, so that a lens of 10 in. focal length might only require a camera extension of 5 inches. This property of the lens did not, however, receive much attention, and only when telephotography became popular was it brought to the front. Lenses having the same property are now on the market, being sold by the Busch Optical Company under the name Bis-Telar.
In photographic work it is often found necessary to use a lens of shorter or longer focus than the one usually employed. Any lens can be converted into a lens of shorter or longer focus by placing an appropriate lens co-axial with it. The amount of change of the focal length varies with the separation of the lenses. The focal length, F, of a combination of two lenses of focal lengths f1, f2 at a distance d apart is (f' f2) / (f' + f2-d)
Thus if we have convergent lenses of focal lengths 10 and 5 inches at a distance apart of 1 inch, the focal length of the combination is (10 x 5) / (10 + 5-1) in., i.e. 3 4/7in.
The lens employed for altering the focal length is called a Supplementary Lens, and sets of such lenses suitable for use with lenses of common focal lengths are made by several opticians. They are sold under the names "Portrait attachment," "wide-angle attachment," etc., and, since they are cheap, are very useful to the worker who does not wish to buy a large number of expensive lenses.
The ordinary Telephoto combinatior consists of a positive and negative lens, the latter being placed between the positive lens and the plate. Such a combination is shown in Fig. 20. The position of the negative lens has well-defined limits. It must never be nearer the positive lens than the difference of the focal lengths, and never farther away than the focal length of the positive lens. Thus supposing we have a positive lens of 8 in. focal length, and a negative lens of 3 in. focal length; then, to use this pair as a telephoto combination we can place the negative lens anywhere between 5 and 8 inches from the positive lens. The formula given above applies to such a combination, due regard being paid to signs. If the positive and negative lenses are complex, then d is the distance between the nearest principal planes of the lenses. The most useful thing to know when using a telephoto combination is the amount of camera extension required for a certain magnification. The extension required can then very readily be found, for if the magnification is n, and the focal length of the negative lens is f, it is (n-1)f.
It will be seen from Fig. 26 that the function of the negative lens is to make the angle of the cone of rays transmitted smaller. This alteration of the angle of the cone of rays without corresponding alteration in the camera extension has a very important effect when the lens axis is not horizontal as in photographing some elevated architectural detail. For full information on this point the reader is referred to the chapters on Architectural Photography.
For the more advanced study of photographic optics the reader may be referred to Photographic Optics and Colour Photography, by Lindsay Johnson; or to the work on Optics by Lionel Laurence.