CALCULATING DISTANCES IN The rule for finding distances when reducing is : Divide the longer base of the image to be reduced by the longer base of copy desired, which will give the number of times of reduction ; to this add I and multiply by the focal length of lens used. The result will be distance between lens and object, and this distance divided by the ratio of image to object will give distance between lens and plate. As an example, assume that a picture measuring 8+ in. by 6f in. is to be re duced to a base of 2+ in., using a lens of I I in. focal length. Then 81 zf = 3f, number of times of reduction; (3* + i)x II = 48*, dis tance from lens to picture; 48 ÷ = distance from lens to plate. It will be seen from this that the working distance of lens is increased from I x in. to 141 in., and therefore the working aperture of the lens is reduced ; thus fp I becomes fi14+, and this factor has to be taken into account when estimating the exposure.
The accompanying table supplies the data for reducing or enlarging a copy, when the focal length of lens is known. The figures given are for linear enlargement ; for example, 3 in. to 12 in. = an enlargement of four times, or, vice versa, a reduction to one-fourth. It is important to note that when reducing (copying, slide making, etc.) the greater of the two numbers
given in each square of the table is the distance from the original to the lens, and the smaller number is the distance between plate and lens. When enlarging, the greater of the two numbers is the distance from the lens to sensitive paper, and the smaller number is the distance between lens and negative.
The figures in each square in line with the focal length of lens give the distances on each side of the lens for copying same size and for enlarging up to eight times. Assume that a whole-plate print is to be reduced to a quarter of its size and that a lens of 51 in. focal length is to be used. Look for the 4 on the top line (number of times) and for the 5+ in the left hand column (focal length of lens) ; where the two columns meet will be found the numbers 27+ and 6+, indicating that the distance between print and lens will be 271 in. and between lens and plate 6+ in. If, however, a negative is being enlarged four times, 27+ in. will represent the distance between sensitive paper and lens, and 6+ in. the distance between lens and negative. REDUCTION AND ENLARGEMENT: