Patent ID: 8730592

Claim:
An optical system for observing a front object and an approximately lateral object, wherein a first lens group with negative refractive power, a second lens group including a reflective-refractive lens, an aperture stop, and a third lens group with positive refractive power are arranged in that order from a front-object side, the reflective-refractive lens is provided with a first face arranged on the front-object side, a second face arranged on an image side, and a third face formed between the first face and the second face in a circumferential direction to surround an optical axis, the first face includes a first transmission surface formed with a center of the first transmission surface on the optical axis and a first reflection surface facing toward the image side and formed around the first transmission surface, and the first face is an aspherical surface that is shaped to be smooth and concave toward the object side in a vicinity of the optical axis where the first transmission surface is located, with a center of concavity being on the optical axis, and that is shaped to be convex toward the object side in a region farther from the optical axis, where the first reflection surface is located, the second face includes a second transmission surface formed with a center of the second transmission surface on the optical axis and a second reflection surface facing toward the front-object side and formed around the second transmission surface, and the third face is configured as a transmission surface.