A storage unit is configured to store beforehand a table that shows a relation between spread parameters and command values supplied to a control unit to acquire a focused image of an object, and information that designates a command value corresponding to an inflection point on an approximated curve showing the relation that the spread parameters have with the command values, the control unit being configured to control a state of an optical system in accordance with an input command value. The luminance information about the object that lies at a distance falling within a range over which focusing should be detected is acquired, at the position of the optical system, which is obtained by the command value corresponding to the inflection point shown in the table.