Zoom lens, and imaging system incorporating it

The invention relates to a compact zoom lens system that has a decreased F-number and can easily achieve a high zoom ratio, and an imaging system that incorporates it. The zoom lens system comprises a first group G 1 of positive power, a second group G 2 of negative power, a third group G 3 of positive power and a fourth group G 4 of positive power. Upon zooming, at least the first group G 1 , the second group G 2 and the third group G 3 move on an optical axis in such a way that, at a telephoto end with respect to a wide-angle end, the space between the first group G 1 and the second group G 2 becomes wide, the space between the second group G 2 and the third group G 3 becomes narrow and the space between the third group G 3 and the fourth group G 4 becomes wide. The second group G 2 is composed of a negative lens L 21 , a negative lens L 22 and a positive lens L 23 . The zoom lens system satisfies a condition that defines the ratio of the total length (length from the first surface of the lens system to an image plane) to the focal length of the zoom lens system at the wide-angle end.