RMS granularity

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RMS granularity is a method developed by Kodak for measuring the grain of a given film stock at a given exposure density. It stands for the square root (R) of the arithmetic mean (M) of the square (S) (see root mean square) of density variations. RMS values can be more objectively measured through the use of a small aperture (48 micrometre) microdensitometer which scans exposures made at a uniform density. These values can then be mapped out across a sensitometric curve chart, so that the density of a particular film will correlate to an exposure time which is also a point on the RMS curve, and can thus be used to derive the RMS value. Typically, the RMS curve for a film negative will peak at underexposed densities before hitting optimum grain reduction at normal and slightly overexposed densities.

RMS Granularity RATING (or RMS Granularity SCALE): The RMS Granularity NUMBER is not the same as the RMS Granularity RATING. The RMS granularity rating indicates the film's relative graininess. Each successive RMS number represents a doubling of the graininess. For example, a film with an RMS 5 granularity rating is twice as grainy as a film with an RMS 4 rating. Note: RMS granularity ratings of print films and slide films are not directly comparable. As a rule of thumb, you should multiply a print film's RMS number by 2.5 to approximate its graininess compared to a slide film's RMS rating.

The RMS Granularity NUMBER on the other hand is simply the Standard deviation for random-density fluctuations for a particular film."