How is ISO determined for color negative film with separate RGB emulsion layers?
Asked 2/11/2020
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I understand the basic sensitometric method for black-and-white film: find Dmin, add 0.10, take the antilog, then calculate film speed from that value. But color negative film has separate red-, green-, and blue-sensitive layers, and sensitometric curves often show different Dmin values for each layer.
How is a single ISO speed assigned to color negative film when the layers differ? Also, does the same approach apply to Kodak Vision 3 5254 digital intermediate film, or is that type of film rated differently for practical shooting?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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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.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
6y ago
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For color negative film, a single published ISO is still assigned, but in practice it is based mainly on the green-sensitive layer, which is typically the slowest and therefore the limiting layer for speed.
So while each emulsion layer may have its own sensitometric curve and Dmin, the film is not normally given three separate ISO values for still-photo use. The final speed rating is established from sensitometric testing plus practical trial-and-error exposure tests.
For Kodak Vision 3 5254 digital intermediate film, the situation is different: it is not a normal pictorial camera negative film. Intermediate/internegative films are designed for duplication work, with very different curve shapes and exposure goals. Their setup is normally determined by careful testing of aperture, shutter, and filtration, then held consistent so the film can compensate for varying source densities.
That means a standard still-photography ISO number may not be very meaningful for 5254. If you want to bulk-load and shoot it in 35mm, expect to do exposure tests rather than rely on a conventional ISO rating.
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