ADVANTAGES OF THE TELEPHOTO LENS.
By the use of a telephoto lens the photo grapher is enabled to obtain a magnified image of a distant object on the ground glass screen without an abnormally long extension of camera. To realise the importance and advantage of the telephoto lens, the two factors on which the size of the object focussed on the ground glass screen of the camera depends must be clearly grasped. Those two factors are, first, the distance of the object from the camera ; and second, the equivalent locus of the lens employed to form the image on the focussing screen, the size of the object being in the exact ratio of the equivalent focus of the lens used. For example, any object rendered on the screen 2 inches high by a 10-inch lens will appear rendered 4 inches high by a lens of 20-inch focus. Now, sup posing a lens of 10-inch focus is in use, and it is found that the size of the image on the screen is too small, while for one reason cr another it is impossible to approach nearer to the object, the only remedy would be to use a.
lens of longer focus. This would mean that a lens of 40-inch focus and a camera at least 40 if taken with a 10-inch lens. The disadvan tages and costliness of such a method of ob taining an increass in the size of the object are most obvious, and it is these drawbacks which the telephoto lens overcomes. As an example of what can be done with a telephoto lens, a comparison of Figs. 864, 865, and 866 will be instructive. Fig. 864 shows a view of a distant building obtained with an ordinary ]ens ; Fig.
865 was taken from the same standpoint with a low-power telephoto attachment ; while Fig.
866 was secured with a higher power, this being, however, still a small degree of magniti inches long would have to be employed, posing it was necessary to obtain an image on the plate four as large as it would be cation compared with what could actually be achieved if necessary.