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) of density variations. RMS values can be more objectively measured through the use of a small aperture (48 nm) 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.