The luminous rays, in fact, form in the objective a double truncated
cone whose upper base is equal to the diaphragm, and the lower one to
the diameter of the lenses. If the aperture be equal to any diameter
whatever of one of the cones, the result will be the same; but, for
the same period of exposure, it will evidently prove advantageous to
approach the diaphragm. The ratio of the apertures that give the same
results at the optical center or behind the objective is as that of the
diaphragm employed to that of the back lens. If the diaphragm is
one centimeter and the lenses four centimeters, an aperture of one
centimeter in one case and of four in the other will give the same
result.
We shall see further along that it is advantageous to employ apertures
equal to several times the diameter of the diaphragm or lens. Now, from
what we have just said, an aperture, equal for example to four times the
diaphragm, will be only 4 centimeters, while the corresponding aperture
behind the lens must be 16.
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