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.
Lack Of Detail In Shadows. The improper handling of the diffusing curtains to diffuse the light and the improper use of the reflecting screen will cause lack of detail in the shadows. Under-exposure is also responsible for this difficulty. Remember, it is always necessary to give a sufficient amount of exposure to secure on the sensitive plate the detail which is in the deeper shadows. A negative which is under-developed and correctly exposed will also lack detail in the shadows. Such a negative will be improved by intensifying.
Judging Proper Exposure. In order to ascertain the correct exposure it will be absolutely necessary to make a number of tests. If an exposure of two seconds is sufficient for Plain Lighting, where the major portion of the face is in light, then it is safe to give four seconds where the larger part of the face is in shadow. It is advisable to keep a memorandum of all exposures, time of day, conditions of light, complexion of the subject you are photographing, etc. Upon developing the negative, if you find it is under or over-exposed you should govern yourself accordingly the next time you make a negative under the same, or nearly the same conditions. Always make proof prints from each negative made, and note on the back of each proof all the data pertaining to its production, and by filing these proofs you have them for your future guidance.
Reflecting Screen. Little or no difficulty should be experienced in handling the reflector, if the directions given in the lesson proper have been followed. Bear in mind that reflected light should be simply a continuation of the direct source of light. The reflected light should blend the harsh line which sometimes exists between the high-light and shadow on the face. Always have the reflector toward the front of the subject, never directly at the side. By turning this screen first one way and then another, observing on your subject the effect produced, you will soon learn the proper angle at which it should remain. But be careful that the reflected light is not thrown so strongly into the deepest shadow on the rear of the head as to flatten the whole effect.
Background. For Rembrandt Lighting effects the background is best when of a dark shade. When possible, however, it is advisable to place the ground so that the part directly back of the shadow side of the face will receive light from the window. In this way the general balance of the portrait picture will be correct, and the subject will stand out in relief from the background. A shaded background may be employed which will aid in producing this result.
Too Much High-Light On Shadow Side Of Face. The face is turned too much toward the light; it should be turned away from the light until the shadow cast by the nose blends to some extent with the shadow of the cheek. There should be only a faint patch of light on the top of the cheek bone under the eye on the shadow side of the face.
Room Too Narrow. When working in a narrow room, or in confined quarters, it may be necessary to place the subject closer to the window and strongly diffuse the light so as to reduce the harshness which would otherwise result from having the subject so near the light.