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.
Subject Closing Lips Too Tightly. Never tell the subject to close or open the mouth. Rather converse with them and get them to answer some question; or, finally suggest that they pass the tongue over the lips to moisten them. The mouth will then assume a natural expression. The exposure should be made instantly, before the mouth begins to twitch. Under such circumstances the subject will seldom know just when the exposure is being made.
Catch-Light In The Eye Too Large. This will usually occur with subjects having large pupils. It is also caused by turning the face too far into the light. The strong light striking the entire pupil makes it appear large, while if the face is turned a trifle farther away from the light, permitting illumination to strike the iris, there will be a much smaller catch-light. By turning the face from the light this difficulty is overcome entirely. Be careful that the eyes are not directed too high, or too low, nor should they lead the face too far. In cases where the eyes are extremely large they should not lead the face at all, but be perfectly straight. Placing the subject too near the source of light will invariably give an extremely large catch-light, as the catch-light is an exact reproduction of the source of light. Therefore, the greater distance the subject is from the light the smaller will be the catch-light.
S15. Eyes of Subject Appear Staring. - This is usually caused by having the subject look too long at the same object, during the preliminaries of posing or lighting; or the subject may be near-sighted and the object looked at may be too far away from the eyes. Do not tell the subject exactly what to look at, or where to look, until you are ready to make the exposure. There are times, however, when the operator desires to observe the exact apppearance of the eyes on the ground-glass. When such an occasion arises tell the subject to look in the particular direction desired for a minute or so, but under no circumstance have the sitter stare at one particular object for any stated length of time. Watch carefully, and by standing close to, or farther from the subject, who is looking at you, it will soon be easy to judge the proper distance at which the eyes will rest naturally. Also permit the subject to wink as often as desired during the exposure.
216. It is quite objectionable to ask the sitter to gaze at a small object, as the eyes invariably wander from a small object, and thus show movement in the finished picture. If the object at which the sitter is directed to look is of considerable size, it will be much easier to fix the attention upon it, providing, of course, that the object is not too large. The better plan, however, is to have the subject look at the operator, and by continually talking and holding the attention, any difficulty along this line will be entirely obviated.
Properly Illuminating Background. To illuminate the background properly, the end of the ground farthest from the light, behind the shadow side of the subject, must be turned toward the light at such an angle that the strongest illumination will fall evenly over the entire ground. In this way the end of the ground farthest removed from the light will be evenly illuminated, photographing exactly as it is painted. If the background is turned at an angle that will cause it to cast a shadow on itself, it will appear very much darker than when turned directly into the light Experiment by placing the ground at various angles, carefully watching the changes of illumination.
Background Too Sharp. This is caused by either placing the background too close to the subject, or by using too small a stop in the lens. For a cabinet size portrait, using a lens of ordinary speed, such as f-4.5, it will not be necessary to stop down at all. The background should always be at least three feet to the rear of the subject-a greater distance even, will improve the effect
Sun Striking The Skylight. This difficulty is overcome by properly arranging the diffusing curtains on the skylight. If the skylight is so located that the sun strikes it directly at certain times of the day, in addition to diffusing the light with the white diffusing curtains on the skylight, make use of the black curtains on the diffusing screen, using the tan at the top of the screen and the black at the bottom. Usually, the tan curtains will supply all the diffusion required.