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354 PRINCIPLES OF CINEMATOGRAPHY
ments: (1) that the claw shall operate positively in both directions. (2) that the rewind handle shall be easily detachable from the mechanism. (3) that the mechanism release button shall be provided with a positive lock in the 'operating' position. (4) that drive shall be provided to selectively rotate either the supply or take-up spools automatically and according to the direction in which the film is travelling.
Variable Aperture Shutters
It is possible to produce a reasonable 'lap dissolve' effect when only a rewinding mechanism is provided in the camera. However, this effect will not be wholly acceptable since the only process by which it can be obtained is that of gradually altering the aperture of the lens diaphragm and, in consequence, altering the depth of focus of the optical system. 'Lap dissolving' is the term used to describe the effect in which the first scene gradually becomes less dense until it finally disappears entirely whilst, at the same time, the second scene is gradually 'fading in' and, therefore, it may be described as 'the fade out of one scene overlapped with the fade in of a second scene'. Obviously, any mechanism used to produce a 'lap dissolve' will also be capable of producing either a 'fade out' or a 'fade in' since both such effects are combined to produce a *lap dissolve'.
If a camera is fitted with a shutter so designed that its aperture may be varied whilst it is continually rotating 'lap dissolves' and 'fades' may be produced without altering the depth of focus of the lens system since the lens diaphragm may remain at one setting throughout the entire operation. The lap dissolve is produced by smoothly closing the variable aperture shutter at the end of the first scene, stopping the camera when the shutter is completely closed, rewinding that amount of film which passed through the camera whilst the shutter was closing and then, after selecting the required camera angle for the second scene, to start the camera motor once more and smoothly open the shutter blades at a rate equal to that at which they were moved during the final stages of the first scene.
Most 16-mm cameras which are fitted with variable aperture shutters rely upon the operators skill in closing and opening the shutter at such a speed that the two effects will be perfectly overlapped. Other cameras, such as the Maurer '16-mm professional', can be operated with an attachment whereby the shutter blades are automatically engaged to close or open at a rate which is positively coupled to the camera mechanism and therefore,