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.
Exposure. While it is advisable always to expose correctly, it is quite essential that most accurate exposure be given for this style of lighting. Over-exposure to any great extent will give flat effects. This being a broad lighting the shadows are on the side of the face which shows the least, so if properly made there should be no strong high-lights. The broad side of the face should be made up of middle tones, free from either strong highlights or dense shadows. Owing to the fact that the subject is placed farther from the source of light, a trifle more exposure will be required than that for ordinary Plain Portrait Lighting.
297. Do not infer that by diffusing the stronger highlights with the diffusing screen that the general illumination is being reduced, because in reality its effect is thereby increased, as the shadows are lightened on account of the general diffusion of light throughout the room. If an exposure were made without using the diffusing screen, it would require almost double the amount of time to secure detail in the deeper shadows, and this would clog the high-lights. In other words, the delicate half-tones would be over-exposed to such an extent that they would, when the negative is properly developed, be as dense as the highest points of light. By diffusing the high-lights they are softened, the shadows are illuminated, and correct exposure is secured with much less trouble. (The exposure given the subject illustrating this lighting, was 3 seconds, with a Portrait Unar lens and no stop.)
298. It will be seldom necessary to use the reflector, after having used the diffusing screen judiciously, except when the shadows are too dense. Then it will be necessary to throw a little reflected light into the deeper parts of the shadows. Use the reflector only as an assistant to the diffusing screen, remembering that reflected light must be only a continuation of the direct source of light. Therefore, the reflector should be placed at an angle to the skylight, so that the end of the reflector farthest from the light will throw little or no reflected light on the side of the subject's face.
Artificial Means Of Strengthening High-Lights. There are times when it may be desired to produce delicate catch-lights on parts of the face that you will find difficult to obtain in the regular way. These effects may be secured by applying with the finger tip a little vaseline, or cold cream, upon the bridge of the nose, top of the cheek bones, protruding parts of the forehead, and on the tip of the lips and chin. Vaseline or cold cream will have just enough gloss to produce the necessary strength in the high-lights.
This can be easily removed after the exposure is made, by using a soft cloth or towel. This method, however, should be resorted to only when it is desired to produce certain effects, and, of course, with the consent of the subject.