INTENSITY OF LIGHT.
As already stated, the general factor of intensity is that of the lens aperture. It may be explained that every lens has a focal length—that is, it has a point of minimum distance at which an image can he produced. According to the qualities of the lens, so is it possible to use a. stop of certain proportions, the relation which the diameter of this stop bears to the focal length being referred to as its intensity ratio. Suppose, for example, two rooms are compared (Fig. 162), one 20 ft. long, A, and the other 10 ft. long, a, and each is lighted by one window only, 1 ft. square, c. It will be seen that the amount of light received on the wall at the far end of the room will not be the same in each case ; as in the larger room, A, the light rays represented by lines R R have further to travel and have to spread over a larger area than in B.
In fact it will be noted that in the case of B they are covering a space only one fourth the size, and therefore this will be four times as brightly illuminated. So that if their relative light intensities were meas ured, the one would be found to be only a quarter that of the other. Most lenses are now marked with numbers, which express the fraction their diameter is of the focal length ; they usually range from f8 to f64, f8 being -A-th of the focus, and f64 being-hth of the focus. The proportionate exposure required with these stops will be inversely as the square of the diameter, clue, of course, to the difference in the area of the opening. For example, the difference of exposure required with f8 and fl6 will be as the square of S is to the square of 16, or as 1 is to 4.