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. The greatest difficulty experienced in home portraiture is to comprehend the vast difference between the density of light in and out of doors. The variation of exposure is so great that where l-25th of a second would be sufficient exposure for a view out of doors many times this amount would be required in a well lighted room. The misjudgment of time and under-exposure no doubt account for the production of many negatives that lack in gradation and, consequently, show extreme contrasts.
221. Take into consideration that in the open air there is an immense volume of light evenly distributed, while indoors the volume of light is greatly decreased, and the iliumination is also unequal, for the light in the room coming from a small opening or window is concentrated and, consequently, unevenly distributed throughout the room. It is, therefore, necessary, in order to be able to give correct exposure, that you become familiar with the strength of light in the room used. By practice and close observation alone can this be accomplished. Until familiar with the required time for various effects, keep a memorandum of the time of day, prevailing conditions and length of exposure given every subject, being guided by the average results.
222. One point must always be borne in mind - that is, you must time for the shadows. By this is meant you must expose long enough to supply detail in the shadows, no matter how much the high-lights are over-exposed. The general tendency is to under-expose. Never light a subject with strong high-lights and dense shadows, but soften the high-lights by diffusing them with the diffusing screen on the window. When diffusing the high-lights in this manner the illumination of the shadows is very materially aided, for by diffusing the concentrated light coming from the window it is spread over a larger area, consequently illuminating a greater portion of the room, resulting in more illumination for the shadows. If carried too far, however, the diffusion will result in flatness; therefore, aim to diffuse only enough to give softness to the high-lights, as the shadows also can be illuminated by means of the reflecting screen. Because of this, bear in mind that when diffusing the high-lights the shadows are illuminated as well; also, remember that with strong high-lights there will be dense shadows, and that the denseness of the shadows regulates the length of exposure. Soft high-lights and well illuminated shadows, whether obtained by diffusion or direct light, will enable you to work with a more rapid exposure.
223. To determine, approximately, the necessary exposure for At-home portraits, it is advisable, after selecting the room to be used as a studio, to first make an exposure of the interior of the room, taking into the view the window prepared for use. In making this exposure of the interior, time for the shadows at the far end of the view opposite the window. If it requires 40 seconds to produce a fully timed plate of an interior about 14 feet wide, it will require approximately 15 seconds (or a little more than one-third this exposure) to fully time a portrait with the subject placed at half that distance, or seven feet from the light. With the subject about one-quarter the width of the room from the window, or 3 1/2 feet, the time required will be about one-third of the exposure necessary when the subject was 7 feet from the light - or 5 seconds. If a lens that will give a fully timed interior view in less time than 40 seconds is used, less exposure for the portrait will be required. If a fully timed negative of the interior has been produced, you will have a guide for future experiments under like conditions, taking into consideration, however, that the nearer the subject is to the camera - producing a larger image - the longer will be the exposure, and vice versa.
224. When a portrait or rapid anastigmat lens is used an exposure of from 1/2 second to 2 seconds is usually sufficient. With a rectilinear lens, or the lens usually fitted to hand cameras, longer exposure will be required; the exact amount depending upon the speed of the lens, which can only be ascertained by trial.