CLASSIFICATION OF SUBJECTS Exteriors: Clouds with sun directly beyond them as for obtaining the so-called moonlight effects 2M Average clouds in sunlight not in above position . . , 512 New snow 256 Bird's eye, mid-distance, marine and old snow views 128 White subjects, foreground and otherwise 128 Average exterior 64 Dark colored foreground subject as green trees, etc. 32 Portraits: Light complexion 64 Average complexion (that of the average man) . . . . 32 Dark complexion 16 Room Interiors: White walls 12E Average walls 69 Dark walls 32 Dark machinery 16 In order to make still clearer, if possible, the meaning and purpose of the foregoing table suppose that in a certain case on measuring the light its p.o.p. time is found to be 1 second; the actinicity of any subject then would be ex actly as given in the table. This in fact is exactly what the table is, as has already been stated, viz., the actinicity of each particular subject when the p.o.p. time of light is 1 second. In a certain case should the p.o.p. time measure second then it is evident that the light is four times as strong as the conditions provided for in the table and the intensity of any subject must therefore be four times that mentioned in the table. Or according to the rule given, the actinicity given in the fable for any subject, divided by the p.o.p. time of the light, gives the actinicity of that subject.
Note that the average exterior subject, when the p.o.p. time is 1 second, has an intensity of 64 actinos and that, aside from clouds and new snow, no subject has more than double, and only very dark subjects less than half that intensity in the same strength of light.
As a concrete example of the method through out, suppose that it is desired to photograph an ordinary landscape whose actinicity with a 1 second p.o.p. time (the conditions of the table) is 64 actinos. Now on measuring the light the p.o.p. time is found to be 8 seconds, the sun being at the horizon. By the rule : 64 ÷ 8 = 8, the actinos of intensity of the subject. Or by analysis: A p.o.p. time of 8 seconds indicates a light strength only i as great as would be indi cated by that of 1 second, the condition pro vided for in the table, and the intensity of the subject will of course be only i of the table in tensity of 64 actinos or 8 actinos. Now suppose that it is desired to photograph this scene on a plate which has a speed of 4 minutes. Now since 4 minutes, the speed time of the plate, is the exposure with diaphragm number one when the intensity of the subject is 1 actino, the exposure with the same diaphragm when the subject has 8 actinos of intensity will be 1 of 4 minutes or : minute, or as already ex plained, 32 seconds. And if the exposure is
32 seconds with unit scale diaphragm number 1, and it is desired to expose with number 64 the exposure will be 1-,, of 32 seconds or I second. This, on reading, may seem somewhat compli cated or extended but the fact is that the only operations are to divide, in this case, 4 minutes by 8 and the result, 32 seconds, by 64. It is evident that two such simple mental divisions cannot be a lengthy proceeding.
Each worker may alter the factors given in the preceding table in order to ascertain by trial the values which produce results pleasing to his own taste. In fact the consideration of all values quantitatively should make one absolutely free to ascertain for himself the speed of any plate or film used as well as those factors in practice which give pleasing results according to his own ideals. If one does not quickly reach this stage of freedom in his experience, it is cer tain that he has not fully grasped the fundamen tals of the system. It is strongly recommended that the student practice all the experiments out lined, especially those referring to the speeding of emulsions, measuring the light, etc., and also that the unit scale of diaphragms be placed on his lens or shutter so that all work done by the light and the lens upon the sensitive emulsion may be accounted for by the ordinary processes of reasoning and by means of simple units as has so often been explained and illustrated. A comprehension of the convergent theory of light measurement as applied in the f/1 meter and in lens diaphragms and also in all light calculations, is essential in establish ing perfect freedom in the analysis of all light conditions.
The p.o.p. time method of measuring incident light is already somewhat known, being the same as taught in the author's book "Home Portrait ure," where it is mentioned as the " Solio time" of the light, and in his later book — "Steadman's Complete Exposure Method and Home Portrait Helps" where it is spoken of as the "tint time," the "p.o.p. time," or "first appearance time" of the light as in the present work. This measure ment is also used as the light factor in working the author's "Aaba Exposure Scale."