900. Exposure and Regulation of the Stop. Under the usual conditions under which cine matography is practised, the rate at which the pictures are taken strictly limits the duration of exposure, each cycle of the mechanism not allowing the exposure to be longer than two thirds of the cycle, that is about 1/24th second at the maximum (and often only 1/32nd of a second) for films taken at the speed of 16 frames per second, and 1/40-1/50 second for films at the rate of 24 frames per second.
The shutter is usually formed of two or three discs with cut-out sectors, which can overlap, leaving an open sector of variable angle, which in some cameras is as much as In simpli fied instruments, the shutter often has a fixed opening of and the regulation of the amount of light to be passed to the emulsion can then be made only by the lens stop, which thereby plays a part quite different to its normal one in photography, where it should be used only for the purpose of regulating the depth of field, unless the lens is fitted with a neutral grey screen of appropriate density, either alone or combined with a light-filter.' 901. Taking the Pictures. It will be noticed that at the rate of 16 frames per second, the rate of turning of the handle is equal to that of the " quick step " (120 paces per minute). For this reason some operators hum a march tune as a help in keeping time when taking a view_ For turning the handle, the forearm must be nearly motionless, the hand turning round the wrist and not the forearm round the elbow. In general, the camera is supported with the left hand while the right hand turns the handle, that is, unless the left hand is employed in adjusting the panoramic head or the swing around a horizontal axis, both of which opera tions are better done by an assistant.
A fault common to new operators is to exag gerate the duration of scenes in which there is no real movement (landscapes, seascapes, etc.) ; a duration of 15 seconds, that is, 30 turns of the handle, must be regarded as a maximum.
It therefore very often happens that scenes with different lighting are recorded on one film, the limits of each scene being marked by a perforator actuated from outside the camera, so as to permit of each scene being developed.
separately according to its contrast. It is advis able to take at least 20 in. (or about 3 turns of the handle) more than the strip to be used, if it is wished to cut off pieces of film for develop ment tests without encroaching on the scenes themselves. If it is desired to remove the exposed film at once, it is necessary to turn off about 3 ft. extra of blank film (about 6 turns of the handle) in order to be sure that the last useful pictures have entered the receiver.
In amateur cinematography with reversing film, which leaves only little latitude in the choice of the duration of exposure, it is necessary to avoid photographing on one and the same film scenes of very different character, or even scenes with notably different lightings, unless an exposure meter is used for determining the " exposure," i.e. the stop to use.
902. Various Manipulations. Except in the more and more frequent cases when the negatives are developed in continuous machines, as is almost always the case with positive film, development is carried out on frames or drums. The frames on which the film has been wound are completely immersed in horizontal or vertical tanks containing the various baths. Drums with a horizontal axis and on which the film is wound in helix form, dip in the baths only at their lower part and are turned at a uniform rate during the whole operation, the various parts of the film thus dipping in the liquid only at intervals.' When a preliminary development test is made on a piece of film, it must be remembered that differences in the speed of development result from differences in the renewal of the liquid around the film due to differences in the move ment of the film in the bath. It is therefore necessary to ensure that development of the test piece is carried out under conditions as little different as possible from those attending the actual development of the 903. Drying the Film. The film must be freed from adhering water before drying, either by wiping with moist cotton-wool or with a wash leather, or by drawing it between rubber squeegees, or, again, by blowing with compressed air. This is necessary to avoid the presence of drops of water, which will give rise to markings.