Exposure values (EV) are calculated based on illuminance (in the case of anincident light meter), or luminance (in the case of a reflected light meter). Thecalculation formulas used by these light meters contain constants called"calibration constants," which are the values that produce the differences in EVvalues. These constants are defined for specific ranges by each manufacturer.These values ar e what the manufacturers use to set their standard exposurevalues.Note that since this explanation is based on the image reproduction, the termMiddle Tone ” is used in Data Transfer Software (application)