Amateurs are often prone to imagine that there is some marvellous property inherent in portrait lenses, and they frequently purchase and take home with glee a prize found in some pawnshop. The older worker is not to be caught with pawnshop bait. The fact of the matter is, that the general improvements in the anastigmats and the increased rapidity of plates has made the old portrait lenses a drug on the market, and amateurs should be shy of buying such lenses without careful consideration as to whether they would suit their purpose. Any lens may be used as a portrait lens, but it is desirable that as long a focus as possible should be used ; for the further the camera is away from the sitter, the more natural and pleasing will be the ultimate result. Of course, for studio work a large aperture portrait lens by a good maker is often a desirable acquisition, but it must not be forgotten that such a lens requires a studio camera, or at any rate a long-focus camera of the square type. During recent years a number of makers, principally in France and America, have devoted themselves to constructing lenses for portraiture which will give broad and painter-like effects, owing to the fact that microscopic sharpness is impossible with them. The results are certainly pleasing and greatly reduce the retouching necessary on the negative. The Spencer Soft-Focus Lens is a typical and excellent example of this class.
Telephoto lens combinations have been considerably improved during the past few years, and more is known of the principles underlying their successful use. Such lenses, exceedingly valuable as they are, should not be purchased till the worker can make good technical negatives when doing ordinary work. If a telephoto attachment is then required it may be had as a separate fitting, which is placed between the positive lens and the camera, and such an attachment will usually give a great range of magnification. The shorter the focus of the negative lens, the greater the magnification with any degree of camera extension, but the smaller the plate covered. This being the case, the worker to whom cost is of little moment will probably have three tele-negative lenses, all to fit into the same tele-tube, and they should be as follows : * For high-power work, the focus of tele-negative should be 1/4 that of positive lens. For medium and general work, the focus of tele-negative should be 1/3 that of positive lens. For low-power and rapid work, the focus of tele-negative should be 1/2 that of positive lens.
While all these lenses would, perhaps, permit of the same mangification being secured, it is probable that the high power and shorter-focus negative lens would not cover the plate to the margins under about four magnifications, while the low power in which the focus of the tele-negative is half that of the positive lens, would probably cover the plate to the corners with two magnifications. On the other hand, the low power would require greater camera extension, and consequently we recommend for general work a tele-negative of one-third the focus of the positive lens.
During recent years opticians have devoted much attention to a modification of the Tele-photo lens, so that it is now possible to use a lens of long focus, but requiring short camera extension. Of course, such lenses are not variable in focus, but they have the advantage of rapidity and are undoubtedly easier to use than the regular telephoto system. The " Magnar " of Zeiss is of 18-inch focus, works at f/10 and requires a camera extension of 6 inches. The Bis-Telar of Busch, is made in a great variety of foci, and nominally works at f/7, but we ourselves when using this lens always stop it down to f/11 because we wish our results to stand enlargement. Dallmeyer makes an " Adon " of 12-inch focus requiring a camera extension of 5 1/2 inches and working at f/4.5. The best of this type is the Ross " Telecentric," the definition of which at f/5.4 is remarkable. It is made in a number of sizes, the foci varying from 9 in. to 17 in., and in each case the camera extension is only about half the actual focus.
LONG-FOCUS LENSES WITH SHORT CAMERA EXTENSION.
* For Telephoto Calculations, see Appendix.
It must be borne in mind that none of the lenses of this type are rectilinear and care must be taken not to use them on subjects where vertical or horizontal lines come near the edge of the plate.
In telephotographic work the greatest aid to good work and brilliant negatives is a satisfactory adjustable lens hood. This cuts out much of the light excepting that used for forming the image, and the difference in result between negatives taken with and without the hood is hardly credible, except to those who have made the experiment.