the disclosed zoom lens is constructed with a front lens group including , from front to rear , a positive lens unit , a negative lens unit and a positive lens unit having a positive overall refractive power . a rear lens group having a negative overall refractive power arranged on the image side of the front lens group moves axially along with the front lens group in differential relation . the rear lens group is constructed with a negative front lens having a glass relatively large abbe number and a positive rear lens having a glass of relatively small abbe number to achieve good stability of chromatic aberration correction against zooming .