Patent ID: 8071929

Claim:
An automatic focus control unit comprising: a first light-emitting element adapted to emit a laser beam to a sample via a single object lens selected from a plurality of object lenses different in magnification from each other; a line sensor having a plurality of light-receiving elements arranged adjacently to each other, a portion of the plurality of light-receiving elements receiving light reflected from the sample; a second light-emitting element adapted to emit a laser beam to another portion of the plurality of light-receiving elements; a slit member adapted to cut across the line sensor so as to gradually increase an area of the line sensor irradiated with the laser beam emitted from the second light-emitting element; a shifting mechanism operative to relatively shift the line sensor in an arrangement direction of the plurality of light-receiving elements with respect to an optical axis of the reflected light; and a controller adapted to register the plurality of light-receiving elements irradiated with the laser beam emitted from the second light-emitting element and amounts of light received by the plurality of light-receiving elements in association with the selected object lens in a state where the selected object lens is focused on the sample, and control a position of the line sensor based on the registered values when the object lens is used again.