This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Exposure. We now come to the most important item in Autochrome work, and, it must be said, the only stage of the work which presents any difficulty, if the subsequent instructions are carefully followed out. In order to make really good Autochrome pictures, the exposure must be correct. It will first be useful to know that the comparative speed of this plate is about 100 times less than the Eastman's Kodak film, or any ordinary extra rapid dry plate. Primarily, the Autochrome emulsion is one-fifth the speed of Kodak film, then the yellow absorption filter increases the exposure five times, and the color starch grains increase the exposure four times, so that we have 5x5x4 = 100. It will, therefore, be clear, that under conditions in which a Kodak film would be fully exposed in a one-hundredth of a second, the Autochrome plate will require a full one-second exposure. It is advisable at all times to measure the actinity of the light by means of an actinometer, so that some degree of standardization may be arrived at; although so varied are the exposures necessary under various lighting and weather conditions, it would seem to be impossible to lay down a standard exposure scale which could at all times be a reliable guide; but if careful record of every exposure and result is made, it will be found to be invaluable for future reference.
1041. Using a Watkins' light meter, and working in the open air on a sunny day in summer, the speed of the Autochrome plate will equal Watkins' 3, when working with the lens stop f. 8. That is to say, the exposure required for a sunlit landscape will be one-third the time which the meter paper will take to darken to the standard tint. If, however, the sun is obscured, or the object photographed is in the shade, then the plate speed will be Watkins' 2, and the exposure will be half the meter time. Again, if the exposure is made in a well-lighted room or studio, the speed of the plate becomes Watkins' 1, and the time of exposure will be the same as the time which the meter paper takes to tint; the lens stop in each being f. 8. At f. 6 the exposure will be reduced by half, and at f. 16 increased four times.
1042. If Wynn's actinometer be used, the speed of the plate in the open air should be taken as 14, and similar variations of calculation made in order to arrive at the correct exposure under varying conditions.
1043. The extreme thinness of the Autochrome film allows very little latitude in exposure, so that a very slight incorrectness, under or over, will show itself in the quality of the finished result. In a weak light, and particularly in autumn and winter, the ratio of exposures necessary will be considerably higher than those suggested, it being sometimes necessary to increase the exposures as much as six to ten times the meter time. As with ordinary photography, great allowance must also be made in exposure when it is desired to make a large image of the object photographed, necessitating the placing of the camera close to the subject, and greatly extending the focal-length.
1044. In landscape work, it is often advisable to slightly under-expose, particularly in bright summer sunlight. Over-exposed landscapes have gray or pink skies, and while under-exposed views, in which heavy foreground foliage appears, will have an excess of blue in all the greens, slight under-exposure gives a better rendering of the blue of the sky. A deeper yellow light filter, or one made as before suggested, with a slight green tint over one-half, gives a much better rendering of brightly lighted landscapes than Lumiere's normal screen.
Development. We will now give full working details for development and finishing of Autochrome pictures, and while we believe that in the hands of some workers the original Lumiere formula gives the best results, we are convinced that certain modifications will generally be found to be advisable, so that although the following solutions are lettered for uniformity in the same order as Lumiere's, slight but important modifications have been made in the formulae.