RAPIDITY.
Although this subject has been dealt with elsewhere (p. 33), a few further re marks may not be out of place. The rapidity of a lens depends almost entirely on its effective aperture as compared with its focal length. The measurement of the diaphragm is not necessarily the effective aperture, although it will be so in the case of a single lens with the diaphragm in front. Effective aperture may he de fined as the diameter of the bundle of rays which pass through the lens to form the image. Rapidity is also affected in directly by absorption, scattering, and reflection of light inside the lens. Absorp tion may occur through the lens not being absolutely transparent, or through dis coloration. The transparency of the glass will vary with its quality, composition, and thickness, while discoloration may be due to decomposition of the Canada balsam used in cementing, or to a process of oxidation in the glass itself. Scatter ing of light may occur through imperfect polishing of the lens surfaces, and reflec tion depends on the optical structure and arrangement of the glasses and dia phragm, as explained in dealing with flare (see p. 372). The method of caleu
lating the rapidity of any lens from its aperture will be found in the section on "Exposure of the Photographic Plate." To FIND THE EFFECTIVE APERTURE.
The following method is useful when it is doubtful whether the diaphragms are correctly marked or not. Focus for in finity—that is to say, on a distant object ; remove the ground glass and replace it with a thin sheet of metal in which a fine hole is bored with a needle. This hole must be so situated as to be, as near as possible, in a line with the axis of the lens. A lamp is then placed behind the hole so that a diverging pencil of light is thrown on the lens. If a piece of ground glass is now placed in front of the lens, a disc of light will fall on it, varying with the size of the stop. The effective aperture is obtained by measur ing the diameter of this disc. To find the f value, divide the diameter so obtained into the focal length of the lens. For ex ample, if the lens is of 10 in. focus and the disc measures 1.1. in., the stop should be marked f's.