EXPOSURE It is scarcely necessary to emphasise the great importance of correct exposure in negative making ; but it may be remarked that when the plate has been correctly exposed, all subsequent work is comparatively simple and straight forward, whereas with an incorrectly exposed plate all the subsequent operations are difficult and unsatisfactory, and the production of a good print is sometimes impossible.
The correct exposure of a plate depends on four varying factors : the subject ; the light, which varies according to the season, the time of day, and the weather ; the speed of the plate ; and the " rapidity," or working aperture, of the lens.
In the earlier photographic days no attempt was made to work systematically from these four varying factors, but exposures were largely the result of guess-work. About 188o the first attempts were made to systematise the data from which exposures were calculated, Dr. Scott's table of light values, and W. K. Burton's table of comparative exposures for different sub jects beip.g among the earliest examples of their kind. Dr. Scott determined the fact that the value of daylight varied in direct proportion to the height of the sun above the horizon. Con sequently, in equally clear weather, exposures would require to be nearly four times as long in the middle of December as in June ; and also at six o'clock in June exposures would be three times as long as at mid-day. Dr. Scott pub lished a table, about 1883, giving proportionate figures for each hour of the day for the middle of each month. Although these figures were necessarily incomplete, the interval from one month to the next being much too long, this table proved to be of valuable assistance for many years.
Burton's tables provided a series of com parative exposures for different subjects—land scapes, marine pictures, interiors, and portraits— under normal conditions. It gave the exposures under the best possible conditions, and these had to be multiplied by the figures given in Dr. Scott's table for all times excepting mid-day in June. (See also " Exposure Tables.") The most modern method of determining the duration of an exposure is by means of a meter. (See " Exposure Meter.") In process work, and colour work, the lengthen ing of exposure due to prisms, mirrors, colour filters, or ruled screens becomes an important consideration. The larger the prism the more light is absorbed. With a 3-in. prism the expo sure in the case of wet collodion work and enclosed electric arc light is increased by about 2f times ; but this would not be true, for example, with an orthochromatic plate and green filter. Mirrors when in best condition do not greatly affect the exposure, but they will do so as they become tarnished and scratched. The ratio of exposures for colour filters should be determined by photographing black, white, and a scale of neutral greys, which should come alike on all three negatives. Ruled screens increase the exposure by about one-fifth, and as small stops are used the exposure will be much longer than in ordinary negative making, though pro portionately the same. The nature of the " copy " (the original) influences the exposure in half-tone work. A deep red toned print will require the longest exposure.