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362 PRINCIPLES OF CINEMATOGRAPHY
Mask 'C can be used to present two scenes at once the almost classical example being of one person in the lower left hand corner of the frame telephoning to another who, at the same time, may be seen replying from some distant spot shown in the top right-hand corner. Mask 'C can also be used to present changes which take place as an article moves across the frame. For example, if it is required to show a flask of liquid gradually containing more fluid as it passes across the frame, this may be done by first locating the mask as shown at 'C and moving the empty flask from right to left. Then, after filling the flask completely, the whole sequence is shot once more with the mask rotated through 180°. Such a scene should be photographed throughout by using the single frame exposing device and by moving the flask by known amounts between each individual exposure.
A further effect which can be very easily produced without rewinding the film and when the camera is running at full speed is that of a 'spot light' effect upon the centre of interest in subjects which may be in motion or when the camera is 'panning'. The 'spot light effect' is of a circular central section in the frame appearing highly illuminated whilst the surrounding area is only slightly illuminated. This may be produced by placing a rectangular piece of neutral density filter in the matt box and cutting a circular section away from the centre. Obviously, the full exposure will be given to the centre of the picture whilst the edges of the frame will only receive that illumination which passes through the neutral density filter. This process is much superior to other techniques in which a circular opaque mask is used and the film is exposed twice once without the mask, to produce a slight exposure over the entire frame, and again with the mask in position to increase the exposure over the central section. With the neutral density filter in use the whole effect can be produced at once and while action is occurring in the scene.
When titles are being photographed the matt box may be used to produce 'wipes' from one title to the next although, in this case, the entire sequence should be photographed by using the single frame release device and by a camera fitted with a film rewind mechanism. If shaft 'E', Figure 156, is extended beyond the matt box it will be possible to locate shutter 'D' in line with successive graduations 'G' marked off around the framework of the box. The rotation of the shutter in a clockwise direction will cause an increasing area of successive frames to be masked off as the end of the first title is approached then, by rewinding