Early Work In Photography
A Text-book For Beginners

60 Early Work in Photography. becomes an important factor in calculating exposure. It is, therefore, very necessary to know the size of our "stops” or diaphragms, relative to focal length, and, in order to ex- press this, we measure both. Dividing the focal length of the lens by the diameter of the opening of the stop, we place the result as denominator of a fraction, with / as numerator. Thus a one-inch stop in an eight-inch focus lens is f/8 while the same stop in sixteen-inch focus lens would be f/16. As, using a one-inch stop, the image at six- teen inches distance is only one-fourth the brilliancy of that at eight inches distance, it requires four times the exposure; but stops marked with the same focal value require a similar exposure. Or, in other words, the exposure in each case is proportional to the square of the / number. For con- venience the lens-makers mark the /numbers on the stops, Fig. 19. and usually arrange them so that each stop requires half the exposure of the next one smaller, or double the exposure of the next larger. They usually run f/8, f/xx 3, 1/16, f/22, &c. Wide Angle and Narrow Angle. As a matter of fact all lenses are both wide-angle and narrow angle, as well as being both long and sliort focus at the same time. We state this apparent paradox because it is very necessary that the student should fully understand the matter, if he is to be the master, and not the slave of his instruments. Suppose we have here a lens of nine inches focus, throwing an image upon a half-plate (63 x 4% inches). We should call it a medium-angle lens. If, however, we are only using it for a quarter-plate (4% x 3|inches), we should call it a narrow-