The amateur, whose photography is done only during the best months of the year and who confines himself almost entirely to subjects of the same character, frequently attains, but not without spoiling an appreciable number of plates or films at the beginning of each season, a certain skill in judging the exposure, roughly, it may be, but closely enough to enable him to obtain satisfactory negatives.
This is not the case with the photographer, who, from necessity or choice, attempts succes sively a very wide range of subjects, sometimes exterior, sometimes interior, nor with the worker who through force of circumstances has to operate in a climate to which he is unaccus tomed. tinder these conditions, through errors of judgment, the exposures very frequently fall outside the limits of normal exposure, causing a high proportion of useless negatives.
Since the time of exposure is easily ascertain able with a sufficient accuracy for all practical purposes,' the photographer can find no excuse when he makes a large error in the time of exposure through failing to use an exposure meter, the price of which vill not exceed that of two or three dozen plates or films, and may be much less than this.
In order to obtain the maximum benefit it is necessary to choose an exposure-meter based on a logical principle. In the following paragraphs we will describe the principle of the various types of instruments constructed for the deter mination of the time of 324. By exposing a piece of sensitive paper of standard quality to light, and measuring the time taken to produce a tint identical with a standard comparative tint, it czui be assumed, to a close approximation, that the inverse of the time of printing-out is a measure of the intensity of light, at the point at which the paper is exposed (Soleil, 1842; Bunsen and Roscoe, 1862).
devices, combined with a calculator which converts the time taken to print out into the time of exposure, were introduced into photographic practice by A. Watkins (1803).
If the speed of the sensitive paper is constantly proportional to that of the emulsion employed, which assumes that they possess similar chrom atic sensitivities and identical laws of darkening, then the time taken by the paper to reach the standard tint, when the actinometer is placed in the subject perpendicular to the optical axis, will be constantly proportional to one of the times of exposure which would be suitable for the reproduction of a scale of tints, ranging from black to white, placed in the same position with respect to the camera as the actinometer. It is assumed that the scale of tints is kept at a constant distance from the camera (or at a distance sufficiently great as to have no effect on the time of exposure) and that the lens is used always at the same relative aperture. It is an easy matter to calculate the equivalent time of exposure, if any one of these factors be altered, or if an emulsion of different sensitivity be used.
Now, if a scale of tints momentarily attached to the subject is correctly reproduced, then the subject also will most probably be correctly reproduced. In the many cases where the time
of exposure is determined by the intensity of the shadows of the subject, which is most usually the case with photographs taken at a short distance, the correct position for the actinometer will be in the shadow (the shadow of the subject, or, more easily, the shadow of the operator) ; when dealing with panoramic views without foreground or very contrasty subjects, where it is only possible to regulate the time of exposure according to the high-lights of the subject, the actinometer must be exposed to the sun. 1 In either case, as already pointed out, it must be directed towards the camera.
It is obvious that none of the conditions we have assumed is rigorously fulfilled (in par ticular, the sensitive materials vary considerably in sensitivity from one make to) another), yet, owing to the fact that the range covered by normal exposures is a fairly extended one, the indications furnished by the actinometer lead in the majority of cases to satisfactory negatives. Objection may be made even to this method for the measurement of the time of exposure, since in very bad light the actinornetric time, as well as the time of exposure, is very so that wide variations may occur between the beginning and the end of the exposure. It is more especially with interior subjects that this objection is valid, but it may be easily met by choosing an aperture such that the time of exposure will be equal to the actinometer time. The exposure is then started at the same time as the paper is uncovered, and continued until the standard comparison tint is reached, the actinometer then function ing as a light integrator, thus permitting the exposures to be ascertained however much the source of light may fluctuate.
The actinometer is normally used in con junction with a table indicating the relative sensitivities of most plates and films, and a. calculator which converts the time of ex posure determined for any one aperture into the equivalent time of exposure for any other aperture.
These instruments are usually arranged to indicate the minimum time of exposure.
However, in order to allow for the various sources of error, and taking into account the fact that errors on the over-exposure side are less prejudicial to the final rendering than errors on the under-exposure side, it is advisable, for all subjects other than those of exceptional contrast, to double the time of exposure calculated from the actino Do not omit, where it is necessary, to take into account the coefficient of the colour filter employed.
In the Wynne actinometer, the number of divided scales on the calculator has been reduced to two, owing to the choice of a very ingenious method of expressing numerically the speed of the various plates or films. If a certain emulsion is said to have a (mean) sensitivity of F/9o, then the minimum time of exposure for this emulsion on a sufficiently distant subject, with the aperture stopped down to would be equal to the actinometer time. On this