EXPLOSIVE POWDER.—A powder used in the flast-light process of illuminating during exposure. (See Flash-light.) EXPOSURE.—By exposure is meant the exposure to the action of light any sensitive sur face, either in the camera to the image formed by the lens, or behind a negative in a printing frame. The exposure being the most important of all the photographic operations, it is lament able that but little useful information can be given upon the subject. The proper knowledge of the length of the exposure is only attained by practical experience. With such sensitive substances as albumenized paper, or with any other printing-out process, the difficulty of exposure is reduced to a minimum, as the operator can see the image, and by attention stop the pro gress of the action of the light at the right moment required. With dry plates, however, or with any other process in which the image is invisible it is a very different matter The expo sure of the dry plate is the most important point, as unless cor rectly exposed a satisfactory picture can rarely be made. The exposure of the plate is principally governed by the following:— (1) The subject to be photographed, its distance from the camera, and darkness of the shadow detail; (2) the rapidity of the lens, and its working aperture; (3) the actinicity of the daylight; and (4) the rapidity of the plate.
There have been many tables and arrangements invented for calculating the necessary exposure, but some of which are more likely to lead to confusion to the beginner than to aid him.
The actinicity of the light varies so with the different days, or even times of the day, and some subjects require so much longer exposure than others, that this information can only be got by practice. There is, however, a law relating to the comparative exposures required with different stops or diaphragms. The stops are usually marked with their fractional value, found by dividing the focal length of the lens by the diameter of their apertures. If all other conditions be precisely the same, the exposures required are proportional to the squares of the denomin ators of these fractions.
In exposing a plate, probably the best guide is the image upon the ground glass focusing screen. If the same kind of plate be always used, the operator, by examining the appearance of image, will soon be able to judge the correct exposure almost immediately. The following table might be useful as giving some idea of the proportional exposures necessary with a moderately quick plate and working with a lens aperture of f/16 with a good light.
Sea and sky. second Animated scenes second Landscape, distant second Landscape, little foreground i second Landscape, dark fo!iage foreground 2 seconds Buildings... 2 seconds Interiors, well lighted 3 minutes Interiors, badly lighted 45 minutes Outdoor portraiture 5 seconds The measurement of the light value is a very difficult proceeding, and cannot be accurately determined by exposing a piece of sensitive paper.
The rapidity of plates is also very confusing. Warnerke's sensitometer is usually employed, and plates are sold by manufacturers of various sensitometer numbers, as " 3o times," " 6o times," etc., intended to represent the number of times the plate is more sensitive than a wet collodion plate. But no reliance at all can be placed on these, some experiments proving nearly all to be incorrect, and some not possessing one-half the sensitiveness they were stated to have.
The following tables by Professor Burton and S. W. Very may also be fonnd of use in esti mating comparative exposures:— In the exposure of bromide paper it is usual to make a few experiments upon small pieces of paper. When once the correct exposure has been obtained it is possible to expose any number correctly, provided the same negative be used, placed at the same distance from the same light, and provided artificial light such as a gas flame be used.
In exposing platinotype or carbon paper, actinometers will considerably aid the operator in judging the exposure time.