Ken Cameron, Sound and the Documentary Film, Sir Isaac Pitman & Sons, 1947


Now, as may be seen from the specimen prints in Fig. 75, when conditions are ideal the negative spread which results in an audible c.p.s. tone will be cancelled out by the spread on the positive. Thus if all the prints are projected through a 400-cycle band-pass filter, which removes all the original 9000-cycle tone, the print which gives the lowest possible 400-cycle content gives the optumum negative and positive densities for that particular emulsion and development.