STOPPING DOWN.
Although the equality of illumination and depth of focus in the telephoto lens is improved by stopping down the positive, great caution must be observed in the closing of the aperture, as should the aperture be much smaller than of its focal length (fl 72), diffraction takes place, spoiling the sharpness of definition in the picture. Where, however, the actual To meet a steadily growing demand, the principal lens makers are now nearly all making telephoto attachments to suit their positive photographic lenses. The old established firm of Dallmeyer has done much to advance and popu larise telephotography, and no treatise on the subject would be complete with out mention of their unique "Adon" telephoto lens, which, with other useful objectives for this class of work, is described in the section on Lenses. Fig. 868 shows Dallmeyer's Moderate power Telephoto lens, which is pro- , vided with an interchangeable High oer t The - Becit-Steinheil series of Low, Standard, and High-power Telephoto Attachments call for special mention on account of their great perfection, the wide field of application which they render possible, and the excellent work which has been accomplished with their use.
The Beck " Multifex" lens, shown in position by Fig. 869, has a focussing mount and . gives a wide range of magnification, being also available for ordinary work. The very latest advance in telephoto lenses is a low power attachment by Goerz, specially designed for subjects requiring the shortest possible exposure. This is especially useful for Natural History photography and similar work, or for securing street scenes, processions, etc., from a moderate distance, when the light permits.