1. An apparatus, comprising: a mechanical shutter, for providing a predefined exposure of an image, said predefined exposure is defined by a global reset time and a closing time of the mechanical shutter; an image sensor with a rolling shutter capability, responsive to said predefined exposure, for providing an image signal comprising calibration and actual image signals by a rolling readout of said image, wherein the calibration image signal is provided by the rolling readout of a predetermined portion of the image using a predetermined calibration area of the image sensor during a time interval between times corresponding to minimum and maximum tolerance values of said predefined exposure for determining, using a predetermined criterion, said closing time of the mechanical shutter using said calibration image signal for compensation of the actual image signal.