An apparatus comprising an interface to a lens iris and a processor. The processor may be configured to present to the interface a signal configured to adjust the lens iris in response to a luma value received from an image sensor. The processor may be configured to adjust coefficient values for the signal. The coefficient values may be adjusted to converge a luma difference value to a set point. The processor may implement rules to determine the coefficient values. The rules may determine the coefficient values based on flexible relationships between input values based on the luma difference value and adjustment values for the coefficient values.