A digital still camera includes a lens system and an image sensor. The image sensor receives object light through the lens system to output an image signal of an object, wherein exposure control and autofocus control of the lens system are carried out according to the image signal. The image sensor photographs an image of the object after the exposure control and the autofocus control. Auxiliary light is applied to the object with an illumination light source for the autofocus control. A second exposure value is determined by use of a first exposure value determined according to object brightness measured while the illumination light source is inactive. Exposure is carried out to measure object brightness by use of the second exposure value while the auxiliary light is applied, for determining a third exposure value for use in the autofocus control with the auxiliary light.