318. Insufficient exposure always results in a thinner negative than would have been ob tained with correct exposure, with lack of detail in those parts corresponding to the darker parts of the subject, and with increased contrast between the images of the shadows and high lights. As we shall see later, it is only in certain special cases that it is possible to compensate for under-exposure by prolonging development: for this purpose the negative must be consider ably under-exposed. Increasing the time of development of a negative in which the shadows are under-exposed and the high-lights correctly exposed can only exaggerate still more the contrasts between the shadows and the high lights, these latter merging into the lighter half-tones unless appropriate correction is after wards made.
Over-exposure yields a much denser negative than one which has been correctly exposed and developed under identical conditions, and the contrasts, especially in the half-tones, is reduced. To obtain the same degree of contrast it would be necessary to prolong deveb prnent, which would result in a further increase in density of the negative. Their liking for a " nice " nega tive, even though it yields poor prints, causes a large number of photographers to cut short the development, in order to obtain an image in which the mean density approaches that of a correctly-exposed plate developed to the re quired degree of contrast.
319. Factors Affecting Time of Exposure. The limits of normal exposure depend on the illumination of the its distance from the camera, the relative aperture of the lens, and the sensitivity of the emulsion, allowing for any colour filter in use. This correction is applied by multiplying the exposure necessary for the plate without filter by the filter factor corresponding with the conditions under which it is used (s,§ 211 and 213).
The illumination of the subject depends, as we have already seen (§,§ 287 and 288), on the height of the sun and the atmospheric condi tions. 2 When, however, the subject is not situated in open country, the extent of sky which can be seen from the position occupied by the subject, and the diffusion of the light by neighbouring objects, both modify the illumination to an appreciable extent. In an interior, for example, the illumination is much more efficient if the walls and hangings are bright and of colours which are actinic to the type of emulsion in use.
The intensity of the image varies only slightly with the distance of the object from the lens when this distance is very large, but it rapidly becomes less as the object is brought nearer and nearer. To compensate for this reduction of intensity, it is necessary, all other factors ator n in the fraction 'in expressing the relative aperture ; in other words, to the square of the F number (§, 7i). The lens stops are usually so graded that, with the same reservation which we have just formulated, the exposure must be doubled as the next smaller stop in the series is used.
It is necessary to stress the fact that in a camera possessing a variable speed shutter, the diaphragm must be used merely to regulate the depth of remaining the same, to increase the time of exposure (§ 91). The table given below shows, in the case of a flat subject, the relative times of exposure for different values of the scale of the image (§ 62) and for the corresponding dis tances between the subject and camera, the distances being expressed in focal lengths of the lens, which is taken as the unit of length.
In the case of objects or portraits, these factors should be considerably increased, for small areas of shadow, which it is not necessary to reproduce in detail when photographing a distant object, become so much more important in a photograph taken at a short distance that a further adjustment of the exposure is required to bring out the details clearly. For example, when working at ro focal lengths as compared with a great distance, the best will be obtained by multiplying the exposure not by I*2 but by about 30, the exposure being then given for the shadows and not for the high-lights.
Finally, as already stated (§ 90), the intensity of the image is proportional, other conditions remaining constant, to the square of the relative aperture. The times of exposure should there fore be proportional, assuming the reciprocity law 2 to hold good, to the square of the denomin Different exposures may somk,qinies suit one and the same subject better according to the interpretation desired by the photographer. The optimum exposure will not be the same, for instance, for a landscape with figures, according as it is regarded as open air por traiture or as a landscape with living figures.