In a zoom lens (30) comprising, from the object side to the image side, a
front lens group (12) and a controllable lens group (24), the
controllable lens group comprises two lens elements (25, 26) having
different dispersions and being movable with respect to each other so as
to perform a focusing action. One of the lens elements corrects for the
dispersion of the zoom lens. Preferably, the zoom lens comprises at least
one folding mirror.