Patent ID: 7907335

Claim:
A focus-adjusting unit disposed on an optical axis between an objective optical system that collects light from a specimen and an image-forming optical system that images the light collected by the objective optical system at a prescribed position, comprising a front optical system and a back optical system, sequentially disposed along the optical axis direction from a front side, with the objective optical system at the front side and the image-forming optical system at a back side; and a lens driving part that relatively moves the front optical system and the back optical system in the optical axis direction, wherein the front optical system and the back optical system have focal lengths of different sign and focal lengths of substantially equal absolute value, and wherein conditional expression (1) below is satisfied: Hb 1 ×Hf 2>0 (1) Where Hb 1 is a back principal point of the front optical system and Hf 2 is a front principal point of the back optical system.