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.
Handling The Diffusing Screen. With a little experimenting any person may soon become able to properly handle the diffusing screen. The conditions under which each individual worker will operate necessitates a trifle different handling of the screen. Remember, however, that the nearer the screen is placed to the subject the greater the diffusion of light, while the farther it is placed from the subject the greater is the volume and strength of the light. The screen should be placed in position, with all the curtains drawn so as to completely cover it. Experiment with the screen at various distances, watching the changes of light diffusion. Then open, or separate, the curtains on the top row, parting them from the center, first about ten inches, or more if needed. The light coming through this opening will supply the high-lights. If the general diffusion is not sufficient place the screen closer to the subject, or farther away, according to the effect it is desired to produce. Follow this line of experimenting until the results wanted are produced. After a little experience it will be a very easy matter to handle the diffusing screen effectively.
Using Reflector. The greatest difficulty in using the reflector is the possibility of throwing too much reflected light on the shadow ear and back portion of the face, thus flattening the effect. The reflector should be placed at an angle of 45° from the skylight, the end farther from the camera being turned first one way and then another, until the desired amount of roundness has been secured. Remember, the strongest reflected light should fall on the front of the face, where the high-light begins to blend into the shadow. Reflected light should be simply a continuation of the direct source of light. Anything further than this will tend to produce flat effects. Strong high-lights often are reduced in contrast, as compared to the shadows, by light being reflected into the shadows, as this raises the detail in the latter to a higher key.
211. It is Important that the reflector be placed at the proper angle, for if badly placed, the portrait, instead of being round, will appear flat. The reflecting screen, while placed on the shadow side, must be situated somewhat in front of the subject, to reflect light onto the front of the shadow side of the face, thus permitting the light on the rear of the cheek to blend off into shadow. Before attempting to make a regular sitting it will be wise to practice with the reflector, noticing very carefully the various effects it produces when placed at different angles.
Posing The Subject. Remember, the more simple the pose the better. Be careful that the shoulders are square, and that the body does not lean forward too much. On the other hand, exercise care that the body does not lean back too far. Have the subject sit erect, yet comfortably. Place the hands midway between the knees and the hips, as this will give a far better effect than any other position. By all means avoid having the shoulders appear round or sagged. Proper results are seldom obtained by placing the body squarely toward the camera. This is permissible only for very slightly built persons. When the subject is placed squarely in front of the camera, it is always advisable to turn the face to one side. Never permit the face and figure to assume the same angle of position. The more easy and natural the pose, and the less you do toward altering the general position of the subject, the better.