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

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LENSES 125 (diverging), the stops being in front of the positive lens. The fact that only two single lenses are included renders this combination more rapid than it otherwise would be, since the loss of light is less. The screw with the milled head is for altering the dis- tance between the two lenses, and thus it is apparent that the focal length of this lens can be altered considerably. There is in fact no limit to the size of the image that can be produced: if the necessary amount of camera extension can be had, the same lens can be used for obtaining pictures from cabinet to life size. The focussing, so as to obtain the required result so far as diffusion goes, can only be learned by practice. It is best to focus some object sharply on the screen, this object being placed an inch or two farther off than the object it is intended to photograph. That good results can be obtained with this lens may be seen by consulting Photography, January 31st, 1903. Arrangements have also been made by means of which the necessary diffusion of focus with the ordinary portrait lens can be obtained by the operator while he is looking at the image on the screen. The method of doing this is illustrated in the next diagram (42), which is an illustration of a Cooke portrait lens adapted for this work. By means of the pulleys shown, a certain amount of rotation can be given to part of the lens, and the necessary diffusion obtained. The diaphragm can also be worked in a similar manner, so that it is not necessary to come to the front of the camera during any part of the focussing operations. Fig.41.