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.
Diffusing Screen. As the light from the flash-lamp is very concentrated, there is a strong tendency toward harshness. In portrait work, therefore, by the use of a cloth screen between the lamp and the subject, or even a group, any amount of diffusion may be obtained. For this screen, a light-weight, bleached muslin or lawn should be used, containing little or no starch and it should be not less than three feet square. Common cheese-cloth will be too thin unless very sharp lightings are wanted, while too heavy cloth requires a large charge of powder, and gives too much diffusion. Accompanying the Nichols' Flash-Lamp is a folding, white muslin-covered umbrella which will be found very suitable for all purposes. For At-home portrait work, a frame built of light wooden strips (window stops), covered with thin muslin will be found very cheap and convenient. This screen should not be less than five feet high and four wide. The bottom of the screen should be four feet from the floor and it should be set at an angle of 45 degrees from the perpendicular.
495. In a screen of this size the sitter is separated from the flash, which will be found very advantageous at times when working in one room. It would be convenient to have the base or support of the screen provided with casters so that it may be moved around as easily as the lightest background. Instead of moving the sitter to produce the desired effect in lighting, you leave the sitter undisturbed and adjust the lamp and screen instead.
496. The screen should be two feet from the lamp and at such an angle that the rays of light from the lamp to the sitter pass through it as nearly perpendicular as possible.
Reflectors. Reflectors to illuminate the shadow sides of the subject should be exactly the same as for ordinary daylight work. As a rule, the reflector should be placed from three to five feet from the sitter on the side opposite the lamp. Do not place the reflector directly at the side of the sitter, at an angle which will reflect the light on the front and side of the subject as well. A safe rule to work by is to place the reflector at as near as possible the same angle from the sitter as that at which the lamp is placed. The light will then be reflected onto the shadow side of the sitter's face at the same angle that the direct source of light from the lamp falls on the light side and will not have the effect of flattening the shadow side of the face by producing a high-light where there should be a shadow.
498. For producing Rembrandt effects, the reflector should be somewhat nearer the sitter than in making Plain Lightings. If you do not use a diffusing screen, the reflector should be considerably nearer than when the diffusing screen is employed.
499. In photographing a person wearing glasses, use no reflector unless a Rembrandt Lighting is made. The use of a reflector in making a Plain Lighting is quite likely to produce a bad reflection in the glasses.