Patent ID: 7817345

Claim:
A zoom lens comprising, in order from an enlargement conjugate side to a reduction conjugate side: a first lens unit of negative refractive power; a second lens unit of positive refractive power; a third lens unit of positive refractive power, the third lens unit having at least three lens elements; a middle lens unit including at least one lens unit; and a last lens unit of positive refractive power, wherein all the lens units, except the first lens unit and the last lens unit, move during zooming, and wherein the following conditions are satisfied: f 3 /fw >4.0 f 3 /f 2>2.0 fw/Bf <0.8 where f2 is a focal length of the second lens unit, f3 is a focal length of the third lens unit, fw is a focal length of the entire zoom lens at a wide-angle end, and Bf is an air-conversion length of back-focus.