An optical system includes: a first lens having refractive power; a second lens having refractive power; a third lens having refractive power and comprising an image-side surface which is convex in a paraxial region; a fourth lens having refractive power; a fifth lens having refractive power and comprising an image-side surface which is concave in the paraxial region; a sixth lens having refractive power; and a stop disposed in front of an object-side surface of the first lens. The first to sixth lenses are sequentially disposed from an object side. A radius of the stop SD and an overall focal length of the optical system f satisfy: 0.2