302. Practical Uses of Flashes. It is proposed to deal more especially with the application to portraiture, for, as we have already seen, in the absence of moving subjects, a much better actinic value is obtained if the magnesium is burned in the form of ribbon.' The frequent unskilful use of flashes leads sometimes to the belief that it invariably results in bewildered expressions, or closed eyes, and, in the majority of cases, complete absence of modelling. Moreover, when using powders, the danger from fire and the inconvenience of the smoke are frequently overestimated, whereas by taking a few simple precautions both may be easily avoided.
The dilation of the pupils, so commonly found in portraits taken by beginners, occurs through taking the photograph in a dark or poorly-lighted room. In accommodating them selves to the bad light, the pupils become very much enlarged and give, in consequence, an expression of fear to the faces of the sitters. This expression is not caused by the flash, but is a faithful representation of the strained look which everyone acquires in semi-darkness. When working at night, the lights should not be turned out, except, of course, such of them as come within the field of the lens. This obviously makes it necessary to cap the lens until just before the ignition of the powder, and to re-cap it immediately the flash has finished. If no other light is available, the beam of light from an ordinary pocket flash lamp may be turned on to the eyes of the sitter, in order to bring the pupils to their normal size.
Another difficulty which arises with the use of a flash is the dosing of the sitter's eyelids due to the dazzle of the light, but this occurs only when the flash has been of too long a duration. A study which should be of great value to the portrait photographer has been made by J. I. Crabtree (of the East man Laboratories) of the reflex movements of the eyelids. In Fig. 164 is reproduced a cinema tograph film, which was taken at the rate of 32 images per second (twice the normal speed) in a lighted room, just at the moment of igni tion of a charge of flash powder, which lasted for about one-sixteenth of a second (corresponding to the two over-exposed images). It will be seen that the eyelids have not begun to close until after the flash has finished, and then the right eye, which was nearer the flash, closed first, to re-open about one-sixth of a second later.' If, therefore, a flash not exceeding one sixteenth of a second is used, the only risk of taking the model with the eyes closed is the coincidence of the flash with a blinking of the eyelids, a coincidence which might occur just as easily during the release of the shutter when taking a photo graph by daylight.
From among the different methods which may be used to ignite the charge, it is preferable to choose those which do not attract the attention of the sitter, avoid ing, therefore, the use of a fuse.
303. The speed of combustion of a powder depends primarily on its composition and its state of division, but it is also controlled to a certain extent by its state of preservation, the weight burnt in a single charge, and by the distribution at the moment of ignition.
Powder which has been kept in a damp place will only ignite very slowly, and will give a low actinic value, owing to the fact that the magnesium powder has been oxidized on the surface or that the combustion agent is damp.
The time of combustion always increases with the size of the charge, hence the need for extremely quick-burning powders when dealing with large groups (banquet halls, etc.), which require large charges. Crabtree has found that with a charge of 75 gr., as compared with one Of 220 gr., using a very rapid powder, the time of duration of the flash increases from 9 to 14 hundredths of a second.
Combustion is always much more rapid if the powder is heaped up in a small cup than when spread out along a channel, in which case the time of the flash may be as much as doubled. This drawback, however, is compensated by a much better completeness of combustion, which allows of the use of a smaller quantity of powder.
304. Numerous methods are available for the ignition of the flash powder, which may be arranged in a heap on a metal tray, or placed along a very shallow channel. In the latter case a bit of gun-cotton, or a strip of paper treated with potassium nitrate (touch paper), is first inserted, and the projecting end is then lighted with a taper held at the end of a stick (the hands should never be placed near the powder at the moment of ignition, unless covered with leather These methods of ignition are only satisfac tory when the flash is used in conjunction with daylight or artificial light, which by themselves almost supply the necessary illumination. When the magnesium flash is the only source of light, it is necessary to use at least two charges, which must be of unequal strength, one of them being used to soften the shadows of the other. Since it is important that the two flashes should take place simultaneously, a special method of ignition must be used, which can be controlled at a distance, preferably at the same time as the shutter.