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Deviation index math
DI = 10 × log (EI ÷ target EI). Every 3 DI doubles or halves the exposure; every 1 DI is about 26 percent.
The formula
The deviation index, DI, compares the exposure index, EI, of an image with the department's target EI for that exam: DI = 10 × log (EI ÷ target EI), using a base-10 logarithm. When the EI equals the target, the ratio is 1, the log of 1 is 0, and DI is 0. Images with more exposure than the target have a positive DI, and images with less have a negative DI.