How to calculate the ISO of colour negative film

Question

I've read about the process in Kodak's publication 'Basic Photographic Sensitometry Workbook'.

My understanding of the process for black and white film, using their example, is that you:

1. Find the Dmin value & add 0.1 - call this A
2. Find the antilog of A
3. Then film speed = 800(mililux seconds) / antilog of A.

E.g

Dmin = 0.8

A = Dmin + 0.10 = 0.9

Antilog of A = 8

Film speed = 800 lux seconds / 8 = 100

So an ISO rating of 100.

However, colour negatives have three layers of emulsion that are sensitive to different colours of light. Looking at sensitometric curves for a particular film, tends to reveal that each colour layer has a different Dmin value.

So how is the ISO calculated in this case?

The reason I ask, is I have a 100' roll of Kodak Vision 3 5254 digital intermediate film (I know not meant for shooting on), but of like to figure out the approximate iso value & bulk load it into 35mm cassettes.

Answer 1

The ISO of color negative film is derived by the formula you posted. Further, the published ISO is derived by trial and error tests based primarily on the green emulsion which is the slowest layer.

Internegative (intermediate)film has radically different curve shapes than pictorial film. The intent is: Avoid the use of cyan filtration in the set-up and self-adjustment of contrast based on the strength of exposure of each individual frame.

For this film, the set-up as to aperture setting, shutter speed and filter pack is based on arduous tests. Thereafter, most frames will be exposed without shifting these setting (exceptions apply). The idea is, thin frames overexpose and dense frames under-expose the copy film. Normal frames fall near the center of the film’s curve. Over-exposed and under-exposed frames fall above and below the center point of the curve.

The steepness of the curve dictates contrast (gamma). The gamma is thus different for altered positions. This scheme is a clever way to auto-correct contrast faults.

Your best bet is to run some trial and error tests to determine a best fit ISO for your set-up. Keep in mind, this is a special purpose film, not intended to be used for pictorial photography.

This likely will be super slow film. Start your test as 32 ISO.