There is no ANSI standard to determine the ISO speed of these films. The speed of motion picture camera films and the suggested filtrations are determined on the basis of practical picture tests to produce desirable density, color and contrast for motion picture workflows. Suitable safety factors have been included to allow for differences in cameras, variation in lighting, etc. The exposure index (EI) values should not be regarded as numbers which express the absolute speed or sensitivity of the film, neither should they be regarded as fixed values which can not be changed if the results of repeated tests indicate the need for such changes.
We use EI (Exposure Index) to rate our films. EI is practically equivalent to ISO when metering but a key difference is that ISO usually represents the film's maximum sensitivity or “speed” to produce acceptable color, contrast and shadow detail overall in its intended process. EI on the other hand represents the suggested “speed” rating suggested by the manufacturer to produce optimal results in its intended process and workflow.
To put it another way, ISO is often assigned based on minimum required exposure to produce “acceptable” shadows detail while EI is assigned based on “optimal” color and contrast. Both are determined through scientific and practical exposure testing but neither one is determined solely by the base emulsion sensitivity without processing since technically all color emulsions contain silver halides with a range of multiple ISO sensitivities that are then amplified through chemical processing which equally plays a role in determining the film’s effective sensitivity to light and overall performance.
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