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.
Stops. top down only just enough to give sufficient sharpness. In portraiture use the lens wide open. If a group picture is being made, after having focused on the center member, it is advisable to use a one size smaller stop than what is required for average sharpness. This will allow for any getting out of line, etc., on the part of any individual member of the group.
513 Amount of Powder to Use. - The amount of powder required for illuminating groups depends upon varying conditions, much the same as timing in daylight. When a muslin screen is used a trifle more powder is required than when an exposure is made without it. The speed of the lens, the size of the stop, the distance of the lamp to the subjects being photographed, the size of the room and the color of the wall paper and ceiling are all important factors. In rooms finished in dark colors, there is little or no reflected light and therefore, at least one-third more powder will be required, otherwise negatives made under such conditions will have an undertimed appearance.
514. Never stop the lens down more than is necessary to cut everything fairly sharp. In portraiture use it wide open.
515. Operate the lamp as near as possible to the object being photographed.
516. For a group in an ordinary sized room with a rapid rectilinear lens and stop f. 11, or U. S. 8, unless the walls are very dark, 1/4 oz. of powder will be sufficient, using the lamp without the diffusing screen. Where the screen is employed 1/4 more powder will be required. Always use a good flash-powder, one that is quick and uniform. The Nichols' Portrait flash-powder, Luxo and Eastman's powders are recognized as good reliable ones and are safe to use.
The Exposure. In working at night it will not be necessary to employ the shutter, but in the daytime in a well lighted room, the shutter and flash must be operated simultaneously. This is accomplished by either running a rubber tube from each end of a double-end rubber bulb - one to the shutter and the other to the lamp - or by using a hard-rubber Y, such as may be secured from photograph dealers. If this is not obtainable you can have the tin-smith make a Y out of brass or copper tubing. In using this Y, connect a short piece of rubber tubing with the bulb on the end of the lower prong of the Y. To each of the upper prongs, attach a long rubber tube, connecting one with the shutter and the other with the lamp. Should there be a difference in the length of the tubing, give the shutter the benefit of the shorter one.
Shutter To Use. Any plain pneumatic shutter that works easily will be found satisfactory, but any of the tension shutters operating with a spring and pneumatic or trigger release, will not answer. Therefore, if you are using an ordinary hand camera and desire to work in daylight, when ready to make the exposure the direct source of light should be curtained down and the exposure made with the bulb operating the shutter in one hand and one for the flash-lamp in the other. Press the shutter bulb first, followed by the flash bulb next, then release the shutter bulb - closing the shutter. By practicing this a little no difficulty will be experienced in making the proper exposure. Care must be taken, however, that the daylight is not given any great opportunity to affect the sensitive plate, for there will be danger of movement of the subject.
519. Before proceeding to arrange the subject and place the camera in position, spread the proper amount of powder on the flash-pan of the lamp. When everything has been adjusted and the subject properly lighted and posed, the shutter closed, plate-holder in place and slide pulled, place the lamp where you want it, light it and step back to the side of the camera so that you may see the picture from the same point as the lens sees it. If using but one bulb you should hold it behind you out of sight. See that the positions are what you want and just before exposure, designate the place upon which you wish the eyes to rest. Do not stop talking but continue some pleasant conversation which will assist in obtaining a more pleasing expression. When the position and expression are what you desire, and if you cannot connect the flash and shutter with the same bulb, squeeze the bulb of the camera first and follow instantly by squeezing the bulb of the flash. Then quickly release the camera bulb as soon as the flash is made, in order that the shutter may be closed at once.