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.
Proper Height Of Camera. The height at which the camera is placed will depend upon the subject, but it may be taken as a general rule that the lens should be on a level with the mouth of the subject, except when photographing standing figures. If the subject is a short necked person, somewhat fleshy, the camera should be below the level, in order to show as much neck as possible. It is also advisable to have the subject lean forward slightly and hold the head erect. The reverse of this is the case when the subject has a long neck, but care must be taken not to go to the extreme, for, usually, a person having a long neck also possesses a long nose, and if the camera is raised too high, the length of the nose will be exaggerated. As you should be governed by prevailing conditions, it is not possible to give any fixed rules covering these points.
Focusing. Always focus with an open lens, and upon the eyes and nose. By focusing upon the eyes and getting them perfectly sharp upon the ground-glass, you are almost assured that the nose will also be sharp. The portion of the face to be out of focus should be toward the rear of the face and head. It is absolutely essential that the front of the face be perfectly sharp. If the ear and back of the head are diffused, the effect of roundness will be increased and the general effect made more artistic. After focusing the face observe the appearance of the drapery. See that the parts nearest the camera are sharp, but not wiry. This may require the tilting of the camera forward or backward, according to the position of the subject. It is not necessary to secure sharpness in the entire drapery but only in the foreground or that portion which is nearest the camera. It is better that the remainder of the drapery blend away into the background.
Stops Or Diaphragms. Never use a smaller stop than is absolutely necessary. If too small a stop is used it will produce extreme sharpness-wiry effects they are sometimes called. In portraiture endeavor to produce roundness and atmosphere. Where small stops are used the background is usually extremely sharp and wiry; therefore, use the largest stop that will permit the retaining of sufficient sharpness to the portrait. It is seldom necessary in portraiture, to stop down the lens; in fact it is not at all desirable to do so unless a very large head is being made. For ordinary cabinet work, however, the lens should be used wide open, or as nearly so as possible. Of course much will depend upon the quality of the lens employed. Some lenses require more stopping down than others, and you must be guided accordingly. All that is really required is to have the front of the face sharp.
Exposure. In portraiture, always and without exception, time for the shadows, allowing the high-lights to take care of themselves. For ordinary Plain Lighting it is better to over than under-expose. Over-exposure of Plain Lightings is easily controlled in development. Under-exposure, no doubt, accounts for many negatives which lack in gradation and are extremely contrasty. Never light a subject with strong high-lights and dense shadows. Soften the high-lights by diffusing them with the diffusing screen. This diffusing of the high-lights also helps to illuminate the shadows, for diffusion of the concentrated light coming from the skylight spreads it over a large area, lighting the greater portion of the room and resulting in more illumination for the shadows. If carried too far, however, the diffusion results in flatness.
156. You should aim to diffuse only enough to give softness to the high-lights. Bear in mind that when diffusing high-lights the shadows are illuminated as well; also remember that strong high-lights are always accompanied by dense shadows, and the more dense the shadows the longer the exposure will have to be. With soft highlights and soft, well-illuminated shadows, whether obtained by diffusion or by direct light, work can be performed with a more rapid exposure.
157. The amount of exposure will depend entirely upon the density of the shadows, but if properly lighted a subject posed to secure a Plain Portrait Lighting will require, under ordinary conditions, an exposure of only a second or two. It is necessary, however, to give an exposure sufficient to secure detail in the shadows. No attention need be paid to the high-lights, as they may be cared for in the developing. It is imperative to obtain on the sensitive-plate an impression of all parts which should show detail in the finished picture, and this impression can be secured only by giving ample exposure. Over-exposure, of course, tends to produce flatness, and this, too, must be avoided. Expose to secure in the negative the effect of the lighting produced.