In principle, the movie maker's negative/positive process is similar to that known to every still photographic worker. As its name implies, development of the camera film is stopped at the negative stage; the positive is produced by contact printing from this negative on to specially made high contrast (and usually fine grain) film, and developed in z positive-type developer.

On the face of it, the negative/positive process would seem simpler than the apparently complicated reversal processing procedure. Insofar as the actual processing of both negative and positive films is concerned, this is probably true. The difficult stage is the printing operation and it is unlikely that the amateur worker will be able to surmount this obstacle satisfactorily. Two problems have to be solved in making prints from movie negatives. The first one is mechanical and revolves around the provision of means for drawing both negative and positive films in intimate contact past a suitable printing aperture and exposing light. Apart from this there must be some means of spooling up both negative and positive film when exposure has been completed.

Negative Scene Changes and Printing Exposure Control

The second problem is optical, and is even more tricky to solve. Generally speaking, the average movie negative is up to 30 or 50 ft. long, depending on the film gauge employed, and within this negative there may be up to 10 or even more separate scenes. It is unlikely that the average negative density for all, these scenes will be the same, and some means must, therefore, be provided for varying the printing lamp exposure accordingly if a uniform print is to be obtained. Since the rate of travel of negative and positive films through the printing gate is set by the driving mechanism, the exposure can only be altered by varying the lamp intensity appropriately from scene to scene. Thus, not only must each scene change be located accurately on the moving negative as it comes within the printing gate, but, at that instant, the lamp intensity must be adjusted to the correct level. What this correct level is can only be determined beforehand by trial and error methods, using small lengths of film exposed and developed under known conditions.

Commercial film processing laboratories get round these problems by using special machinery in which scene changes on the negative are located automatically and the printing light intensity varied appropriately. Skilled operators are able to assess the exposure required for each scene with remarkable accuracy, and where necessary, to provide by means of short exposure strips, a range of picture densities from which the best effect can be selected.

Defects on Prints

Quite apart from the mechanical and optical problems mentioned above, there are other factors which intervene to make things even more complicated for the amateur film processor. No movie maker needs to be told how irritating are the showers of white spots which occasionally appear on his screen, sometimes heavy enough to resemble a veritable snowstorm. These spots can be traced to minute particles of dust on the negative, and during printing, prevent the light from reaching the positive film, so causing small clear patches on its surface. Nothing can be done to cure the trouble once it has occurred. The remedy is obvious. It is for this reason that commercial processing laboratories take such pains to see that the negatives they print are as clean as it is reasonably possible to get them.

A scratch is another irritating defect to be found on a print. Sometimes it is straight, sometimes it wavers and wobbles across the screen. It may be on the emulsion, or it may be on the celluloid side of the print. If on the print, and not too deep, it appears usually as a dark line on the screen; while if on the negative, it appears on the print as a white line. Usually, little or nothing can be done to remove a scratch when once it is formed, and here again prevention is the obvious aim.