Patent ID: 8456751

Claim:
An optical system comprising, 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 first lens unit and the second lens unit being configured to move along an optical axis when a magnification is varied from a shortest focal length to a longest focal length, a largest image height of the shortest focal length being different from that of the longest focal length, wherein the optical system satisfies the following conditional expressions: 1.70≦ Yt/Yw≦ 2.5 Y= 2× f ×sin(θ/2)(85°≦θ≦90°) 3.45≦ bfw/|f 1|≦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.