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.
Strength Of Light. Different brands of plates require softer or harsher lighting of the subject; for instance, Seed, Stanley and Standard plates develop with a tendency toward softness, so the subject should be lighted a little stronger than would be necessary if employing Cramer or Hammer plates. Both of the latter work a little more contrasty, and, therefore, the high-lights may be considerably diffused and they will still develop with plenty of snap and contrast.
Controlling The Light. The next point for consideration is the handling of the reflecting screen, as well as the diffusing screen. It is quite important that the reflecting screen be placed at the proper angle. Often you may observe portraits made after this style of lighting, where the shadow side of the face is as strongly illuminated as the front. This is a mistake, caused by the reflecting screen being placed at an angle which reflects light equally as strong on the cheek as on the fore part of the face.
Diffusing Screen. Adjust the diffusing screen on the window, likewise the curtains, which can be separated at any point desired. Use this screen as a skylight, by raising or lowering it, or opening or closing any portion of the curtains. Drawing the curtains together at the bottom, allowing them to be partly separated at the top, will soften and diffuse the light on the drapery. If this light were not restrained the drapery would photograph more quickly than the face and hair, and would, when developed, either cause the face to appear much darker than the drapery, or if developing for the flesh, you would produce a very hard drapery effect, requiring a great deal of doctoring to make a good negative. Therefore, to avoid unnecessary labor light the subject as you wish the negative to appear when finished.
295. It is a good plan to close the diffusing curtains, thus softening the light, and then separate them sufficiently to supply the necessary "catch" or high-lights. You may be led to think that because you have diffused the strong lights by closing the diffusing curtains this will prolong the exposure. Such is not the case. On the contrary, it tends to shorten the time, for should an open light be used without the diffusing screen, the shadows would be denser, requiring a long exposure to fully time them. While timing for the dense shadows the high-lights are sacrificed and "choked." By diffusing you equalize the light and illuminate the shadows, and as time must be given for the shadows they are not as dense after diffusion; consequently, less time is required for the exposure.
Illustration No. 18 See Paragraph No. 297.
Reflecting Screen. The reflecting screen is used as an assistant to the diffusing screen. It aids in illuminating dense shadows, and must not be used too near the sitter. The nearer the reflecting screen is to the light the stronger will be the illumination reflected, and vice versa. Therefore, place the screen as near the light as is found necessary to supply the proper illumination to the front of the face. Gradually turn the end of the screen nearest the subject at an angle from the light, thereby reducing the strength of the reflected light - which illuminates the denser shadows - from this end of the screen. The angle at which the reflector should remain depends entirely upon the effect produced on the face of the subject.
297. Should the rear of the head, or the shadow portion of the Rembrandt Lighting, be illuminated to the same degree as the front of the face, you would experience the difficulty known as blocked or clogged shadows. Note in Illustration No. 18 (floor plan), the position of the reflecting screen. Observe that it is not parallel with the window, but at an angle of about 45°. The correct angle can only be judged by the strength of the light.
298. Carefully watch the light as it is reflected upon the subject; then, using the end of the reflector nearest the camera as a pivot, gradually turn the opposite end away from the sitter, but not so far that no light is reflected. It is necessary to produce enough detail to gradually blend the high-lights off into shadow. Only a little practice is required to thoroughly master the manipulation of this screen.