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.
Too Strong Top-Light. This is caused by the subject being seated too near the side-light; and possibly the top curtains are not pulled down far enough on the skylight. Watch for the shadow cast by the nose. When this shadow falls directly beneath the nose too much top-light is being used. When too strong a top-light is used, even when it falls at the correct angle, heavy shadows may be cast under the eyes. This will be especially true if the subject is sitting too far under the skylight. To overcome this, diffuse the light with the diffusing curtains on the skylight. Place the subject so that more light will come from the front. Tipping the diffusing screen a trifle forward at the top will assist in overcoming this difficulty.
Too Strong Side-Light. If the subject is placed too far from the side-light, the shadow cast by the nose will fall too straight across the face, indicating that the light comes too much from the side. If the angle of light is correct and the side-light is too strong, soften it down with the diffusing screen, by moving the screen closer to the subject.
Diffusing High-Lights. When the high-lights are strong the shadows will be very deep, and the result is a very contrasty lighting. To overcome such contrast the strong light must be diffused. Do this with the white diffusing curtain on the skylight, assisted by the reflecting screen. You may find it necessary to change the distance of the diffusing screen and reflector from the subject. You may also find it necessary to use the dark curtains on the diffusing screen. If with the dark curtains drawn there is too much diffusion, separate them at the points where more strength is desired. It is seldom necessary to use the black curtains in connection with the tan, as the tan curtains usually give sufficient diffusion. Learn to see the change, of light on the subject's face, and with the curtains on the diffusing screen soften or increase the light until the desired result is produced. Exercise care not to over diffuse.
Heavy Shadows On Front Of Shadow Cheek. A heavy shadow will appear on the shadow cheek if the subject and camera are placed parallel, or nearly so, with the light; or if the reflecting screen is placed at such an angle that it reflects the strongest light on the back of the head and ear of the subject Too strong a light will also produce this effect. As the aim should be to have the subject surrounded with as much light as possible, it will be necessary to have the light come from the front to a considerable degree. By placing the subject just beneath the top-light the full benefit of all the light entering the room will be received. The subject will be surrounded with more light from the front, while the shadows will be soft and quite easy to control.