# How to calculate the ISO of colour negative film

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.

• I think it would be best to just do some tests. Set up a controlled studio shot and shoot at several ISO settings then dev in "the proper?" chemistry. Find the best ISO and further test to adjust dev times. kodak.com/gb/en/motion/products/lab_and_post_production/… Commented Feb 11, 2020 at 20:22