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62 PRINCIPLES OF CINEMATOGRAPHY
focussed upon some object in the foreground, will not 'resolve' clearly the objects at a greater distance. This condition is known as lack of 'depth of focus' or 'depth of field'. The appearance of such a picture, on the cinema screen, is of a sharply focussed object of interest, plus a background which is often so badly out of focus that it appears as a series of black, grey and white circles of light festooned across the scene. Such an effect is not very objectionable in black and white pictures, in fact, for some years many cameramen considered this to improve the picture by throwing the centre of interest up in relief and by 'removing' all definition from the unwanted background.
Whether this effect really was good art or not will never be decided, but, it is quite certain that a similar result, produced on colour film, is definitely objectionable. Unfortunately, all the colours in the background form into bright circles rather like bunches of coloured balloons. The coloured circles do not remain apart from each other, but overlap at the rim and produce a third colour to add to the confusion. Obviously then, the only alternative to such an effect is to close the lens diaphragm and so increase the depth of field of the optical system. To compensate for the reduction in exposure it may be possible to increase the shutter aperture, Such a move can only be carried to a certain limit, beyond which the remaining opaque section in the shutter would no longer provide a 'closed' period sufficiently long to permit the film to be moved forward. However, if the normal exposure when the shutter opening is only say 140°, is arranged to give an adequate negative when the lens aperture is high, it would then be possible to reduce the aperture, thereby increasing the depth of field, and to compensate for this reduction in exposure by opening the shutter aperture to the full 170° period.
The well known 'fade out', during which the scene becomes progressively darker until it is eventually opaque, may be produced by two methods. Firstly, by gradually closing the lens diaphragm but, as explained previously, such a move would cause the depth of field to be progressively increased during the operation. An alternative method, which does not alter the depth of field, is to provide a variable aperture shutter which may be gradually closed until all the light is prevented from reaching the film. Taking this last suggestion one stage further, we are able to provide a means with which to produce the 'lap dissolve' transition. A lap dissolve is the name given to the transition in which the first scene gradually becomes less dense, until finally it disappears entirely, whilst, at the same time, the second scene is gradually 'fading in'. A more