Abstract: Provided is an observation optical system including: an objective lens; an image inverting unit; and an ocular lens, the objective lens including, in order from object side to observation side: a positive first lens unit; a negative second lens unit; and a positive third lens unit, wherein the first lens unit consists of a positive front unit and a positive rear unit separated by an air interval, which is largest along optical axis, wherein the second lens unit moves to have a component orthogonal to optical axis for image blur correction, and wherein a lateral magnification of second lens unit, a curvature radius of a lens surface of second lens unit closest to observation side, a curvature radius of a lens surface of third lens unit closest to object side, the air interval between front and rear units, and a focal length of first lens unit are appropriately set.

Abstract: Provided is an observation optical system including: an objective lens; an image inverting unit; and an ocular lens, the objective lens including, in order from object side to observation side: a positive first lens unit; a negative second lens unit; and a positive third lens unit, wherein the first lens unit consists of a positive front unit and a positive rear unit separated by an air interval, which is largest along optical axis, wherein the second lens unit moves to have a component orthogonal to optical axis for image blur correction, and wherein a lateral magnification of second lens unit, a curvature radius of a lens surface of second lens unit closest to observation side, a curvature radius of a lens surface of third lens unit closest to object side, the air interval between front and rear units, and a focal length of first lens unit are appropriately set.

Abstract: An infrared optical system, which can obtain excellent image even when used under a severe condition, is easily and cheaply produced by molding. The infrared optical system, is provided with a configuration of 2 groups 2 lenses having the first lens L1 and the second lens L2 sequentially from the object side, the first lens is a convex aspheric lens of which convex surface protrudes toward the object side, and the second lens is a convex meniscus lens of which convex surface protrudes toward the object side, the infrared optical system is formed by using chalcogenide glass such that the extreme values of the surface shape of the first lens L1 and the second lens L2 are not formed at the lens surface including the outer portion of the lens effective diameter.