In field work, more especially in connection with architectural studies in confined spaces, wide-angle lenses are often necessary, and much attention has been paid to this branch of lens construc tion both before and since the advent of the anastigmat. Of late years wide-angle lenses with comparatively large full apertures —very useful for focussing in dark interiors—have been made by Taylor, Taylor and Hobson, whose VII. B series, superseding their previous "Primoplanes," open to f/6.5 and at f/32 give sharp definition throughout a field of oo°. The Wide-Angle Aristostigmat of H. Meyer, the 4 in. model of which covers a half-plate at f/9, is another successful lens of this type. For the particular purpose of Aerial surveying, but also very useful for many other classes of work, a new wide-angle Ross Xpres, em bracing an angle of 8o° and covering sharply from centre to mar gin at the very high aperture of f/4, is a remarkable achievement, the report of the National Physical Laboratory on this objective showing extraordinarily good corrections. Where an abnormally wide-angle lens is needed the Goerz "Hypergon," which consists of two hemispherical lenses, and has a frontal star diaphragm to equalize the light, may be used to cover an angle of 135°. Modern Telephoto lenses are, as previously indicated, very different from the early variable power type, and much better adapted to latterday requirements. The magnification is small, at the most X3, but this is compensated by critical definition enabling enlargement to be carried out to the same extent as with negatives made with ordinary objectives. For the modern tele photo lens is corrected throughout as carefully as the best or dinary anastigmat, being in fact itself an anastigmat, fully achro matized, free from curvature, spherical aberration and coma, and, within its limits, non-distorting. Also, as now made by half-a dozen leading opticians, it is rapid enough for all practical pur poses, even putting aside the latest "ultra-speed" models in which an aperture of 1/3.5 has been reached. The only drawbacks are bulk and somewhat restricted covering power, but these do not constitute a high price to pay for a lens which will do very nearly all that the best ordinary lens will do at half the camera extension required by the latter. An advanced lens of this type is the Ross Three Power "Teleros" which gives an image almost three times as large as that given by an ordinary lens from the same standpoint, and works at 1/6.3. In the Two Power Teleros the aperture has been widened to 1/5.5. There is also a Teleros of 4o in. focus, working at f/8, which is specially constructed for photographing cricket and other sports. Messrs. Dallmeyer, in addition to their "Grandac" and other variable power telephoto lenses, make a number of fixed-focus objectives giving magni fication from X2 to 23 and working at apertures from 1/3.5 to f/7.7. The "Ultra-Speed Dallons" are only made in focal lengths up to 12 in. for covering quarter and 5X 4 plates, but there is a very useful series working at f/5.6, the 24 in. model of which requires an extension of only I2 in. from the back cell to the focussing screen, and covers a whole-plate. Messrs. Taylor, Tay lor and Hobson's "Cooke" Telephoto Anastigmats, Messrs. Wray's "Plustrar," the Zeiss "Tele-Tessar" and the Voigtlander "Tele Dynar" are other fine examples of fixed-separation telephoto lenses. The "Cooke," like the "Dallon," is made with apertures of both f/3.5 and f/5.6, but with the former in two focal lengths, 8 in. and io2 in., only.
In colour photography a high order of colour correction is necessary and for three-colour "process" and other colour work in which very great accuracy is essential it is desirable that the correction should be carried to the furthest practicable limit. This is done by means of special computations and glasses in apochromatic lenses made by Ross, Taylor, Taylor and Hobson, Zeiss and others. With a view to complete correction for colour a few quartz lenses have been made by British, French and Ameri can opticians, and further progress may be expected.
Lenses for cinematography receive mention in the article on MOTION PICTURES, but passing allusion may be made to them here since those employed in cine-cameras are photographic objectives pure and simple, made in most cases to standard formulae. Some
extraordinary results have been attained in these tiny lenses, the Dallmeyer "Dalmac" having been adapted to cinematography in the 16 mm. sub-standard size in a variety of models, one of which has a focal length of I in. only, with an aperture of 1/1.5. A number of tele-cine-lenses are also available.
Very short focus photographic lenses are also used in photo micrography, but are not so suitable for work at high powers as microscope objectives in conjunction with suitable eye-pieces.
An unusual short focus lens, of especial interest commercially, is that employed in the "Graflex" automatic film camera used for photographing meters in the telephone exchanges, etc.
Certain commercial filters, such as the Ilford "Gamma" and the Imperial "Impan," give accurate rendering of all colours when used with panchromatic plates from the same source of supply. The "Impan," by attaining this result, when used with Imperial "Special B" panchromatic plates, with an exposure only 2-i times that required without a filter, has set a new standard. Literally hundreds of scientific treatises and experiments, many culminating in research developments for which scores of patents have been granted to British, French, German and American inves tigators, have been contributing factors to the present stage of all but perfect success in use of filters. Today, we are unques tionably on the eve of very widespread and successful use of colour in all forms of photography.