Zoom lens and projector using zoom lens

A zoom lens comprising, in order from a magnifying side thereof, a first lens group having a negative refractive power as a whole, a second lens group having a positive or negative refractive power as a whole and a third lens group having a positive refractive power as a whole, wherein changing the magnification of the whole lens system thereof is attained by configuring such that the first lens group and the second lens group are made to move on an optical axis in such a manner as to move in a direction from a magnifying side to a contracting side over a range from a wide angle end to an intermediate area and to move in a direction from the contracting side to the magnifying side over a range from the intermediate area to a telephoto end, and the third lens group is made to move on the optical axis in such a manner as to move in a direction from the contracting side to the magnifying side over a range from the wide angle end to the telephoto end, and wherein the following conditional expression (1) is satisfied with respect to a power set to the first lens group and the second lens group, the following conditional expression (2) is satisfied with respect to a power set to the third lens group, and the following conditional expression (3) is satisfied with respect to the size of an airspace on the contracting side of the third lens group at the wide angle end:

• −0.9<fw/fI-II<−0.5 (1)
• 0.35<fw/fIII<0.55 (2)
• 1.7<dwIII/fw (3)
• where
• fw:
• composite focal length of the whole lens system at the wide angle end
• (focused at a magnifying side object distance of 1700 mm from a lens surface of the first lens group which is situated closest to the magnifying side);
• fI-II:
• composite focal length of the first lens group and the second lens group at the wide angle end
• (focused at the magnifying side object distance of 1700 mm from the lens surface of the first lens group which is situated closest to the magnifying side);
• fIII:
• composite focal length of the third lens group;
• dwIII:
• airspace between the third lens group and a cover glass of a light bulb at the wide angle end.