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.
Producing The Dividing Line. In order to produce the dividing line, the subject should face-with the nose almost parallel with the side-light-toward the center of the room. The camera must be placed at such an angle that it will be turned partly toward the light. In this position one-half of the face will be entirely in shadow, while the other half will be well illuminated. The light and shadow should meet on the nose, and the light should blend slightly over the nose onto the shadow side, falling on the cheek bone directly underneath the shadow eye.
Producing High-Light Under Eye On Shadow Cheek. If the face of the subject is turned too far from the light, or if the subject is too far from the source of light, or too near the center of the light, it will be impossible to secure the high-light on the shadow cheek. Move the subject nearer the side-light, at one end of the room, turning the face until the proper high-light is secured on the shadow cheek. If the light is obtained at an angle of 45°, and the head is turned properly away from the light, this high-light on the top of the cheek bone, beneath the eye, will be secured.
Illuminating Shadow Side. If not enough detail is present on the shadow side of the face, not enough reflected light has been used. Place the reflector nearer the subject, and by proper and judicious handling of the diffusing screen and reflector, the proper illumination and detail in the shadows is sure to be obtained. Practice and close observation of the effect produced, coupled with a little experimenting, will readily enable one to overcome this difficulty.
Diffusing Screen. Here, also, only practice and close observation of the different effects produced will enable the operator to overcome this difficulty. Try placing the diffusing screen nearer to the subject. First draw all the diffusing curtains, spreading them over the screen. Then open a small portion of the upper section. Watch the effect produced. If the light is too broad the curtains have been separated too far. Use a narrower opening, then by shifting the screen forward or backward the light can be directed to any point desired. If the catch-light is not sharp enough, draw the black curtains over to the opening. This will make it more decisive. Placing the screen nearer to the subject will give more diffusion; when farther away a broader light is the effect produced. The eye should be trained to see the various effects and enable the instant discovery of any error, or chance for correcting slight imperfections
Strong High-Lights On The Rear Of The Head. Strong high-lights on the rear of the head are caused by having too strong a light coming from the rear of the subject. To overcome this difficulty, diffuse this light, or so manipulate the dark curtains on the diffusing screen as to cut it off. Also, be careful about the placing of the reflecting screen. If it is turned so that the strongest light is reflected on the back of the head and the ear, the reflector must be turned in the opposite direction. Experiment with this until the strongest light is reflected on the front of the face. The strongest reflected light should strike between the nose and the cheek bone, on the shadow side, and gradually blend toward the rear of the head.