Fine grain master positive

Fine grain master positive: motion picture lab slang (? most common in U.S. ?) for a black and white intermediate positive image generated from a negative for the sole purpose of generating additional duplicate negatives.

This intermediate element is exposed and chemically processed to a photographic gamma that is conducive to generating a duplicate negative as close to the original as is possible using photochemical processes.

While fine grain master positives look over exposed and dark, this element contains all information in the original negative "compressed", if you will, down into the toe and straight-line portion of the H&D curve via exposure and chemical processing, so that details will not be lost. The image is "uncompressed" when the duplicate negative is made from this element and the tonal range "expanded" up into the top straight-line and shoulder portion of the H&D curve.

The theory of gamma reproduction states that the original scene is "unity" or "1" gamma. A camera original negative of this scene, if processed to "normal" or suggested photographic speed/density for that stock is .65 gamma.

(A one-stop push is .75 gamma and a two-stop push is .85 gamma -- conversely, a one-stop pull process is .55 gamma and a two-stop pull process would be .45 gamma, but there are physical and aesthetic limits to how low or high you can process film from it's intended aim-point)

In theory, you should be able to multiply the gamma of each element to arrive back at the original values of the scene ("unity" or "1") at the end of the reproduction chain, but this does not work in practice due to a number of reasons; not the least being light scatter in projection that results in a flat, muddy image.

It has been found over the last Century, that a end gamma of 1.40 to 1.60 is pleasing to the eye, so adjustments are made photochemically to boost contrast on the projection element by processing it to enhance contrast.

So, if you have a .65 gamma original negative, you will need a release print gamma of 2.20 or more to obtain a pleasing result on screen.

.65 x 2.20 = 1.43 gamma

Thus,in order to obtain a proper duplicate negative from a fine grain, the chain would resemble this:

.65 orig neg x 1.60 gamma fine grain x .65 dupe negative returns you to a .676 gamma dupe negative, which can be fine tuned up or down with push or pull processing. Of course, this is a greatly simplified model with no provision for accounting for copying methods and other variables.

Also, this equation becomes problematic when copying film of an unknown processing gamma or when making copies of elements already at the suggested projection gamma and requires knowledge and experience to manipulate the copying chain to minimize copying artifacts.

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