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.
Light For Results. When the subject is dressed in white, if the light were not restrained as it falls upon the drapery, the highly illuminated drapery would affect the sensitive plate more quickly than the light on the face. When developed, the face would be much lighter than the drapery; or, if you developed for the face, a very hard, opaque drapery, requiring considerable dodging to make a good printing negative, would be the result. To avoid unnecessary labor, therefore, light the subject as you wish it to appear in the finished picture.
309. It is a good plan to experiment with the controlling of the light, as stated in the "Brief General Instruction," before attempting to make an exposure. The following experiment should also prove interesting and practical:
Diffusing Experiment. Place a vase, cup, or any opaque object, before a window, and about one foot from it; hold a white cardboard back of this object, placing the object between the light and the cardboard. By placing the cardboard about 6 inches from the object, you will find the object will throw a very strong shadow on the card. Now, place a sheet of tissue paper between the light and the object, when the shadow on the cardboard will be very much subdued. This illustrates the effect produced by the manipulation of the diffusing screen. If desired, an actual photographic, test can be made.
311. Make an exposure with the open light (all diffusing curtains having been drawn to one side). In this case give as much exposure as is deemed necessary to secure detail in the deepest shadows, which, under the circumstances, will be very dense and will require about three times the ordinary exposure. Now make a second exposure, having properly diffused the light. Expose sufficiently to secure the proper amount of detail in the shadows, which, you will observe, are not nearly as dense as when the light was not diffused, thus requiring about one-half to two-thirds the exposure necessary with the former plate. Comparison of the two negatives will demonstrate the first to be extremely contrasty, the high-lights will be blocked, and the resulting print chalky. The shadows may have some detail in them, if sufficient exposure was given and the development carried far enough, but the chances are you will stop the development before any perceptible amount of detail has had an opportunity to develop. The reason for this is that the high-lights become so dense you will consider the negative fully developed.
312. With the second negative the conditions are entirely different. The development will proceed gradually, and if the high-lights have been softened by the diffusing screen they will build up in proper relationship to the other tones in the negative. By the time the high-lights have been sufficiently developed full detail will be present in the shadows, and all tones in the negative will be in proper proportion to each other, exactly as they appeared on the subject.