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.
Diffusing Curtains. The direct source of light frequently will be too harsh and you may wish to soften it; therefore, it is essential that the skylight be equipped with a set of diffusing curtains. These curtains are made of thin white muslin, and should be stretched on wires strung horizontally across the skylight. Diffusing curtains soften the light, so that by careful manipulation the strongest high-lights will still retain beautiful half-tones. As muslin may be purchased in yard widths, a 15-foot skylight will require six widths, for each set of curtains must lap over the next set at least 6 inches. Each set of curtains should be divided into three sections, in order that an opening may be made in any place desired.
Diffusing Screen. To control the light still further, it is advisable to prepare a special screen similar to the one shown in Illustration No. 2. The screen complete, including the supporting standards, is 8 ft. high and a trifle over 6 ft. wide. The standard supports are 4 ft. 8 inches high. The screen alone is 6 ft. square, and is attached to the standard, with its lower edge 2 ft. from the floor, by means of a large rod that runs through the two standard supports. The screen is pivoted on these two supports about 4 ft. from the top, leaving 2 ft. of screen below the rod.
72. We recommend, however, a 6x8 ft. screen, with an equal amount of space above and below the pivot and the adjusting rod. The screen will then balance better than if it were 4 ft. above and 2 ft. below the rod. The pivoting or adjusting rod is of one-half inch iron, each end being threaded and fitted with a large thumb screw. On the inside of both sides of the screen frame is a threaded washer. When the screen is tipped to the proper angle, the thumb screw and the rod clamp the sides of the screen between the standard supports and the washers, thus holding it at any angle at which it may be placed.
73. The complete dimensions of the frame are given in the illustration, and these may be followed, except that we advise a 5-ft. standard, with 6 x 8-ft. screen, with the iron rod running through the center of the screen. This will give a better balance and make the screen easier to control.
Illustration No. 2.Diffusing Screen.
See Paragraph No. 71.
74. The screen is covered with curtains of Irish mulle, each two feet in length (each row containing four curtains), and hung on wires attached to the ends of the screen. These curtains can be separated almost anywhere on the frame, thereby permitting white light to fall on the subject exactly where it is needed, closing out direct light which is not desired. All this is done without interfering in any way with the general illumination of the room. This screen is practically indispensable when photographing subjects in white drapery. The tan color of the curtains softens and diffuses the light so evenly, that by careful manipulation the strongest high-lights will contain beautiful half-tones. As it is sometimes difficult to obtain the Irish mulle, a substitute will be found in using thin, white muslin which has been boiled in strong coffee, to which has been added a teaspoonful of salt. Boil the muslin for ten minutes, or until it takes on a light tan color. Do not use heavy muslin, as such curtains would be too opaque. The thin material of a tan color, is better than a heavier material of a white color.
75. The necessary amount of diffusion will depend, to a certain extent, upon the brand of plates used. For instance, Seed plates develop with considerable softness, so it is permissible to use a little stronger and more contrasty light than would be permissible when employing Cramer or Hammer plates. Both of the last mentioned plates will develop with a trifle more contrast than appears on the ground-glass, and it is, therefore, necessary to make due allowance for this by simply diffusing the high-lights.
76. Where a large and strong light is used, in addition to the tan diffusing curtains, it will be necessary to have an additional set of black calico curtains with a dead surface, divided and hung exactly as the former, and on the back of the same frame. The black curtains should be on the side next to the light. The tan curtains can be used for diffusing and the black for excluding the light, should it be necessary to secure more contrast and snappy high-lights, with less diffusion. For instance, when photographing white drapery, if you desire to accentuate the shadows in the folds, the black curtains should be drawn to leave a narrow opening 8 or 10 inches in width, just sufficient to supply white catch-lights.
77. If the portrait is to be a bust, the top row of curtains only need be separated; but for a full length figure, each row should be parted from the top to the bottom of the frame. Of course when white catch-lights are desired, the tan, as well as the opaque black curtains, must be separated; but if you desire to subdue the catch-lights in any portion, close the tan curtains which affect such parts.
78. Another very important point is, that with the curtains separated to supply white light for the high-lights, the closer the screen is placed to the subject the stronger and more contrasty will be the effect, as the direct light is more concentrated. On the other hand, the farther away the screen is placed the greater will be the diffusion and spreading of the high-lights. This diffusing screen is an actual skylight, on a small scale, which can be tilted to any angle, and with which every ray of light falling upon the subject can be controlled. When photographing children, if it be necessary to use this screen at all, only the tan curtains should be employed. After a little experience in shifting and arranging the curtains you will understand the uses of the screen and be able to judge the proper light for securing various effects.
Illustration No. 3. Reflecting Screen.
See Paragraph No. 79.