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.
Securing Sufficient Top Light. As the windows in many homes are quite low it may be impossible to secure a sufficient amount of top light to have the angle of light fall on the subject at an angle of 45°, unless the subject is placed quite close to the source of light. Remember, however, that this will give you more contrast unless you use double diffusing curtains. There must be softness and a gradual blending from high-lights to shadows in all portions of the figure.
Too Much Contrast. Too much contrast is due to one of three things: (a) Having the subject too near the window; (b) insufficient diffusion of the light; (c) lack of proper reflected light. The remedies are: (a) Place the subject far enough away from the window to secure the correct angle of light. As a rule, the distance from subject to the window should be the same as the width of the window, (b) When using a north light there is little trouble in securing the proper amount of diffusion; but if a window is employed with the sun shining on it, it will be necessary to diffuse the light carefully with your diffusing screen, and perhaps also to add an extra thickness of cheese-cloth, in order to have the light under perfect control, (c) Should you be unable to properly soften the high-lights with the diffusing screen, and thus reduce the amount of contrast, it will be necessary to apply reflected light. This, however, must be done very cautiously and care exercised that too much light is not thrown into the deepest shadows on the back of the head and ear. Remember, that reflected light should be a continuation of the direct source of light. The object of reflected light is to simply blend the harsh line which is formed between the high-light and shadow and, in blending, the reflected light should become very weak as it reaches the back portion of the shadow side of the face.
Diffusing High-Ligths. If the high-lights are extremely hard and lack flesh values, the subject has been placed too close to the window, or you have not properly handled the diffusing curtains. The further the subject is away from the source of light the more diffused will be the high-lights, yet placing the subject too far away from the window will cause too low an angle of light. Place the subject only far enough away from your source of light to secure the desired angle of light. A good rule to follow is to place the subject from the window the distance of the width of the window. Then close the curtains on the diffusing screen, and if this is not sufficient to give the desired diffusion and softness, use another thickness of cheese-cloth on the window. Sometimes a portion of a newspaper may be employed advantageously.
329. If your curtains are stained a light coffee color, you will not have any difficulty in obtaining sufficient diffusion, as the brown color of the curtains gives a mellow light. Being very soft in itself, and inclined toward flatness, be careful not to go to the extreme and over-diffuse the high-lights.