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.
Background Too Sharp. This difficulty occurs when rectilinear lenses are employed, as they are constructed in such a manner as to possess extreme depth of focus. In all portrait lenses this feature is eliminated, the depth of focus being greatly decreased, for when the subject is sharp upon the ground-glass the background is out of focus.
277. When the regular portrait lens is not used this difficulty with the rectilinear type of lens may be overcome by placing the background as far to the rear of the subject as possible. Make portraits with the lens as wide open as practical, without sacrificing definition. Another method is to unscrew the back combination of the lens one-half turn. This will secure softness. When small stops are used the background is usually wiry, so employ the largest stop that will permit the retaining of sufficient sharpness in the portrait.
Heavy Shadow On Front Of Shadow Cheek. This heavy shadow will not occur if the subject is placed on a line with the casing of the window, or even back of it. See dotted line A in Illustration No. 12 - diagram of floor plan. If the reflecting screen is placed at an angle that causes it to reflect the strongest light on the ear and back of the head, instead of on the front of the face, effects of this character will be produced. Excessive volume of side light will also produce this effect. By placing the subject almost on a line with the window casing or even slightly back of it the sitter will receive full benefit of all the light from the window - and will be illuminated more from the front than the side, and the shadows will be soft and quite easy to control.
Overcoming Reflection Caused By Sun Striking On The Window. If you have taken the ordinary precaution of stretching cheese-cloth or thin white muslin over the window, and then adjusting the diffusing screen, no trouble should be experienced. At times, however, if the sun strikes directly the light may be so strong as to cause some reflection and harsh high-lights. The only thing to be done is to diffuse the light still more, until the proper softness (not flatness) is obtained. Placing the reflecting screen nearer to the subject will overcome extreme contrast.
Posing The Subject. Follow carefully the advice given in the instruction and do not attempt fancy posing. The more simple the pose, the better. Observe that the body does not lean forward unnaturally, neither should the sitter assume a too reclining position, but one that suggests ease and comfort. By following the instruction regarding the placing of the hands, difficulty with round or sagging shoulders will be avoided.
Catch-Light In The Eye Too Large. This difficulty generally occurs with subjects having large pupils. It is also caused by turning the face too far into the light. If the pupil of the eye receives the full strength of the light it will appear large, while if the face be turned a trifle farther from the window, permitting the light to strike the pupil on the side instead of the front, there would only be a small catch-light. By turning the face away from the light still more, the catch-lights disappear entirely. Be careful that the eyes are not directed too high or too low, nor should they lead the face too far. Extremely large eyes should scarcely lead at all, but be almost straight.