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a point midway between the centre of the screen and the edges. But this is something which need not worry the operator who is screening only a very small picture — it will be quite sufficient if he obtains a finely-focussed centre. It is only on large screens that a soft focus may be seen at the edges when using "average" lenses.
Test-Films for Picture Steadiness
Any film which contains a lengthy "still" sequence and which is also in perfect condition may be used to test the steadiness imparted to the film by the gate tension. Compare the picture with the perfectly-still edge of the screen and note the degree of jumpiness. If you are positive that the film is perfect and the picture jumps, then the fault is mechanical and is caused either by incorrect gate-tension or perhaps by some fault in the intermittent mechanism. A common practice is to put the film out of rack* and observe the lower frame line for steadiness.
Gate-tension should normally be increased until the jumpiness stops and the picture remains perfectly steady, but if such adjustments make no material difference the tension should, of course, be at once slackened off again. If the jumpiness persists, the cause may be either mechanical or due to offset printing of the frame-line in relation to the perforations. But how can the operator tell whether the film or the projector is at fault? The answer is found by making use of one of the new picture-test films, which may be hired from one or two of the specialist film libraries.
These films give a perfectly still image of a number of dots, each of which is actually a small hole punched in the picture area. The holes are made at the same time as the perforations and there is no possibility of them "wandering" as subsequent photographic images may. Any unsteadiness of the dots in relation to the screen itself will prove conclusively that the fault lies with the projector. The trouble could be caused by worn claws or cam, or perhaps by a worn cross and sprocket, etc.
The Buzz Track
A special test-film for correct location of the scanning-beam across the sound-track is also available in the 16-mm. gauge. This film has a blank sound-track, but on either side of the *By means of the framing control.
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