Fine grain master positive

A fine grain master positive is a photographic term. It is also known as a fine grain master or fine grain and is a high-definition black-and-white intermediate positive image generated from a negative for the purpose of creating additional duplicate negatives. This intermediate element is exposed and chemically processed to a photographic gamma that will permit duplicate negatives as close to the original as is possible by a photochemical process.

While a fine grain master appears over-exposed and dark, it contains all of the information in the original negative, compressed into the toe and straight-line portion of the H&D curve via exposure and chemical processing. The image is uncompressed when the duplicate negative is made from the element and the tonal range expanded up into the top straight-line and shoulder portion of the H&D curve.

Theory and classification

In the theory of gamma reproduction, the original image is referred to as "unity", or "1" (i.e., 1 gamma). An original camera negative of the image, if processed to "normal", or the suggested photographic speed/density for that stock, is .65 gamma. As a comparison, 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 is .45 gamma (but there are physical and aesthetic limits to how low or high film can be processed from its intended aim-point).

In theory, it should be possible to multiply the gamma of each element to arrive back at the original values of the image (unity or 1) at the end of the reproduction chain. In practice, this is problematic for a number of reasons, including a light scatter in projection which results in a flat, muddy image.

History

During the twentieth century, it was discovered that an end gamma of 1.40 to 1.60 is pleasing to the eye, so adjustments are now made to photochemically boost contrast on the projection element by processing it to enhance contrast. For instance, a .65 gamma original negative will need a release print gamma of 2.20 or more to obtain a pleasing result on screen (.65 x 2.20 = 1.43).

By way of illustration, to obtain a proper duplicate negative from a fine grain, the reproduction chain might resemble the following: A .65 original negative is processed to a 1.60 gamma fine grain, and from that to a .65 duplicate negative, resulting in a .676 gamma duplicate negative. The resulting element can be fine-tuned up or down with push or pull processing. Of course, the example is simplified and makes no provision for the method used to copy the elements or any of the other variables that would need to be considered. The calculation also becomes problematic when film of an unknown processing gamma or elements that are already at the suggested projection gamma are copied. Knowledge and experience would be required to manipulate the copying chain to minimize artifacts.

External links

* See MediaValue.com's [http://www.mediavalue.com/glossf.htm Glossary of Technical Terminology] for a definition of the term.