Relationship between grain and resolution

As with resolution and other dimensions of film performance, it is possible to quantify grain. Describing it numerically involves using the RMS granularity number. This number is often accompanied by a descriptive verbal classification of each film. Following are the general classifications both in RMS numbers and their equivalent descriptive classifications:
RMS Granularity Numbers and its respective descriptive classifications

  • 5 or less  - Micro Fine

  • 6-10 -  Extremely Fine

  • 11-15 - Very Fine

  • 16-20 - Fine

  • 21-24 - Medium

  • 26-30 - Moderately Coarse

  • 33-42 - Coarse

  • 45-55 - Very Coarse

Grain and resolution might be categorized as separate entities but in reality they are rather interdependent. While there are no absolute international standards by which to measure graininess, (RMS) Diffuse Root Mean Square granularity values are the ones most commonly quoted. Comparing RMS numbers across film categories, such as between color print and slide film, is much like comparing apples and oranges. It is not a meaningful measure in any sense. In order to get any useful information about relative granularity, you must compare apples with apples and oranges with, you guessed it, oranges. Therefore, compare only slide films with other slide films, color print films with other print films, and so on.

Virtually anyone should be able to note the difference between an image classified as coarse grain from one classified in the micro-fine range, provided magnification and contrast levels for the two images are comparable. It is another matter finding visual grain differences in negatives or slides where a relatively small RMS granularity spread exists. For example, two negatives produced from films with RMS numbers of 6 and 8, respectively, should not display noticeable grain differences. It is only when RMS numbers differ by more than four points that finer or coarser grain becomes apparent. Having said this, I hasten to add that recent dramatic strides in photo technology have resulted in several new crystal and dye configurations totally revolutionizing thinking about grain structure in Elm. No longer is it enough to think of grain merely in terms of mass. We must also consider the shape and orientation of the crystals. Grains are no longer homogeneous or monofunctional. They may contain several layers, each performing a separate function. It is important, then, to examine the new grain technology in some detail.

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