CALCULATION OF CONJUGATE Foci.
Let a = the distance of object from lens, b = distance of focussing screen or image from lens, and f = focal length or equivalent focus of lens.
Then 1 — 1 1 or (a + = a h.
f a If the image of an object is x times smaller than the original on the focussing screen, a is + 1 times the focal length of the lens, and b is equal to the focal length + of the focal length. For ex ample, if the image of a yard measure is 3 ill. long on the focussing screen, with a lens of 12 in. focus, a = 13 x 12 = 156 in., and b = 12 4- = 13 in. This rule may obviously he employed for en larging, reducing, and copying, by simply transposing the terms. Thus, the above instance will give the necessary distances for easel and negative, to enlarge times, if for focussing screen is read negative, and for object, easel. To make this clearer, the rule for enlarging may be given. To find the distance from the negative to the lens, divide the focal and amount of spherical aberration present. The shorter the focal length,
the greater the depth of definition, which also increases as the aperture diminishes. A small amount of spherical aberration is occasionally advisable, since it increases the depth of focus ; it must, however, be very slight in extent. Curiously enough, the finer the lens, and the more highly corrected it is, the greater care must be taken in focussing.
length by the number of times of enlarge ment, and acid the focal length. For ex ample, with an s in. lens, and enlarging 4 times, the required distance would be + 8 = 10 in To find the distance from the lens to the paper, multiply the focal length by the number of times of enlargement, and acid the focal length, which, in this case, would mean (S x 4) + 8 = 40 in.