A zoom lens has a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, which is moved during zooming, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power, which is moved during zooming, arranged in the mentioned order from the object side to the image side. The zoom lens satisfies the following conditions: 0.15<|f2 |/ {square root over ((fw.times.ft))}<0.27 and 5.4021 f3/fw<10.00, where f2 and f3 are the focal lengths of the second and third lens units respectively, and fw and ft are the focal lengths of the zoom lens at the wide angle end and the telephoto end respectively.