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

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LENSES 121 the stops may thus be found by dividing their diameter into the focal length of the lens. A little thought will make it quite apparent that the intensity of light upon the plate must depend directly upon the area of the stop used, i.e. upon the square of the diameter, and inversely upon the square of the distance of the plate from the stop i.e. --. ri 7i~^ Now in the in- r L (rocal length)^. tensity ratio the square root of these quantities is used. Hence, in order to compare the times required for two intensities, the ratio between their squares must Jbe taken. Thus a lens working at //16 requires, not twice but four times the exposure of one working at //8. Such an intensity as //16 would be found quite high enough for general landscape and architectural work. Since the photographic value of the light received depends upon its intensity, and in all good lenses the intensity depends only upon the stop used and the focal length of the lens, it should be apparent that all such lenses, when working at the same / value, have the same speed. The term Depth of Focus is frequently used with re- ference to lenses. Scientifically speaking there is no such thing as depth of focus for a lens system. The position of the image as measured from the lens alters as we alter the distance of the lens from the object, and hence two objects at different distances cannot be equally in focus on the screen at the same time. Of course when the distance of the objects from the lens is very great compared with the focal length of the lens, the difference in definition may become so slight as to be practically imperceptible. Hence we may define this " Depth of Focus " as the ability which the lens possesses of rendering the images of objects situated at different distances from the camera equally sharp on the focussing screen. A short focus lens is of course far less influenced by a slight difference in position of objects than a long focus