Optical system, image capturing apparatus, distance measuring apparatus, and vehicle-mounted system

An optical system that forms an image of an object, comprising:

• an aperture stop;
• a first reflection surface;
• and
• a second reflection surface,
• wherein:
• the aperture stop, the first reflection surface, and the second reflection surface are disposed in order from an enlargement side to a reduction side,
• an area of the first reflection surface is larger than an area of the second reflection surface, and
• an expression below is satisfied:
• 95<∠QPR<120,
• where a reference axis is a path of a reference ray that passes through an opening center of the aperture stop to reach a center of a reduction plane, P is the opening center, Q is an intersection of the reference axis and the first reflection surface, R is an intersection of the reference axis and the second reflection surface, and ∠QPR is an angle (deg) between a line segment PQ connecting the opening center P and the intersection Q and a line segment PR connecting the opening center P and the intersection R.