Imaging device and autofocusing method

Abstract

In a digital video camera according to one aspect of the invention, a black level of a video signal is specified during an initialization process. An autofocusing process obtains an AF evaluation value and a brightness evaluation value at each current position of a compensator lens and updates a maximum AF evaluation value and a corresponding position of the compensator lens based on the obtained data. The autofocusing process also detects an AGC gain, computes an offset value of the AF evaluation value from the detected AGC gain, the obtained brightness evaluation value, and the specified black level, and calculates a threshold value from the computed offset value and the updated maximum AF evaluation value. When the obtained AF evaluation value is less than the calculated threshold value, the lens position corresponding to the maximum AF evaluation value is specified as the focus position. When the obtained AF evaluation value is not less than the calculated threshold value, on the other hand, this series of processing is repeated with a position shift of the compensator lens by a preset number of steps. This arrangement ensures efficient and precise hill-climbing autofocus control in view of the potential influence of high frequency noise in the digital video camera or another electronic camera.