a telephoto zoom lens having four lens groups of which the 1st , counting from the front , has a positive refractive power , the 2nd has a negative refractive power , the 3rd has a positive refractive power and the 4th has a positive refractive power . the 1st , 2nd and 3rd lens groups form a varifocal section , and the 1st and 2nd lens groups axially move in opposite directions to each other when zooming . the amounts of movement of the 1st and 2nd lens groups relative to each other are thus controlled to achieve a shortening of the total length of the lens system .