Rear focusing zoom lens

A zoom lens comprising, in the order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, said second lens unit being made to move along an optical axis to effect zooming, said fourth lens unit being made to move so as to compensate for the shift of an image plane resulting from the zooming and being made to move for focusing, said third lens unit including a bi-convex lens and a negative meniscus lens having a concave surface facing the object side, and said fourth lens unit including a negative meniscus lens having a concave surface facing the image side, a bi-convex lens and a positive lens having a convex surface facing the object side, wherein in said fourth lens unit an air space is arranged between the negative meniscus lens and the bi-convex lens, as well as between the bi-convex lens and the positive lens.