The advance of photography : its history and modern applications (1911)

132 THE ADVANCE OF PHOTOGRAPHY Thus for a half-plate the lens has an equivalent focus of 16", but the camera requires an extension of only 8J" for objects at infinity. Since this lens works at //7, it can readily be seen that the exposure required is sufficiently short for this lens to be used for practically all classes of subjects. A very useful device for those who wish to keep the weight of their photographic apparatus as small as possible, and yet be able to obtain good-sized pictures of more distant objects, is the extension lens which can be obtained in connection with the ordinary Cooke lenses. These extension lenses are intended to replace the back lens of the Cooke lens. Thus, suppose the ordina^ lens to be in use with a half-plate camera, and therefore to have a focal length of 8" ; then by means of one of these extension lenses its focal length can be brought up to about 12.3". It must of course be borne in mind when using these extension lenses that the rapidity of the lens system is in all cases altered. In the above case, the ordinary lens works at //5*6, while the lens with the extension works at //ll. In other words the latter combination requires about four times the exposure of the original lens. Further, it is necessary to note that such additional lenses are not intended to enable one to use a larger-sized plate, such as can be done with a lens of focal length equal to that of the new combination, but simply to give a more detailed representation of some particular object or objects. As a rule these extension lenses increase the linear dimensions of the images 50 per cent, as compared with those obtained witn Fig. 48.