An optical system includes, in order from an object side, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power. The optical system satisfies: 1.70≦Yt/Yw≦2.5, Y=2×f×sin(θ/2) (85°≦θ≦90°), and 3.45≦bfw/|f1|≦7, where fw is the shortest focal length, ft is the longest focal length, f is an arbitrary focal length that satisfies fw≦f≦ft, θ is an angle between the optical axis and a principal ray of an off-axis light flux incident upon a lens surface closest to the object, Y is a formed image height of a ray incident at the angle θ, Yw is the largest image height at the shortest focal length, Yt is the largest image height at the longest focal length, f1 is a focal length of the first lens unit, and bfw is a back focus at the shortest focal length.