Patent ID: 7660045

Claim:
An object lens, comprising: a first optical system that obtains a magnified image of an object; a second optical system that guides dark field illumination light to the object; a barrel that contains the first optical system and the second optical system and has an optical path around the first optical system for the dark field illumination light, the barrel having a hold member that has a cylindrical surface and holds the first optical system; and a shield mechanism that is disposed on the optical path and that varies an incident area of the dark field illumination light to shield the dark field illumination light, wherein the shield mechanism includes a plurality of shield plates layered in the direction of an optical axis of the first optical system, the shield plates being opened/closed by rotating about the optical axis so as to vary the incident area of the dark field illumination, and wherein each of the shield plates includes: a first fit portion that has a ring shape and fits the surface of the hold member so that each of the shield plates can be opened/closed; and a second fit portion that fits an adjacent shield plate and causes the shield plates to be rotated together while the first fit portion fits the surface of the hold member so that the shield plates are opened/closed.