When a brightness level of an object is below a predetermined level, an ND filter is removed from an iris opening to fully open the iris opening, and when the brightness level of the object is above a predetermined level, the ND filter is placed to the iris opening to fully close the iris opening. When the ND filter is moved between the fully-opened state and the fully-closed state, an exposure change amount corresponding to an amount offsetting the exposure change amount due to a state transition is generated by controlling iris blades.