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26 III. 16-MM FILM AND ITS CHARACTERISTICS
Television cameras and recording machines, especially those for recording from video monitor tubes, often use film in 1200 foot rolls. 400, 800, 1000, and. 1200 ft. are common lengths in use in film laboratories where copies of films are made ; 1600 and 2000 ft. are lengths that are used infreqently even in film laboratories.
Although extreme care is used in manufacture and in supervision and inspection during manufacture, it must be realized that the quality of perforating and slitting varies from manufacturer to manufacturer and, for a particular manufacturer, varies from time to time. The tools and dies used are subject to wear; the quality delivered depends to a great degree not only on how well the tools were designed and made but also on how wTell they are maintained. To make it possible to check mechanical variations, manufacturers of film maintain accurate manufacturing logs and also mark each can of film shipped with sufficient identifying code numbers to aid in tracking down any possible deviation from standard deviation in manufacture.
General Comment on Emulsions
The control of emulsion quality exercised by film manufacturers is of a high order; it is quite rare that defective or inferior emulsions of regularly manufactured products such as Kodachrome, reversal film, duplicating negative, duplicating positive, and release fine-grain positive raw film get into circulation. This does not mean that all lots of a particular type of emulsion are alike ; emulsions vary from lot to lot to an even greater extent than does film base. The sensitometric characteristics of a particular lot depends not only upon its characteristics at the time of manufacture and test by the manufacturer, but also upon the changes in characteristics as a result of storage during the interval between manufacture and use. Temperature is a very important storage condition; generally speaking, a constant temperature of approximately 50° F. is most suitable for most films. As a result of improved manufacturing processes, the storage characteristics of films have been improved greatly in the last few years. Not only has there been considerable improvement in the base, but also in the length of time that a film may be stored before use (as measured by a predetermined permissable change in sensitometric characteristics, such as fog, etc.) has been increased to the point at which manufacturers can safely guarantee their products for a period of at least one year even on high-speed, negativetype films.