Patent ID: 8922905

Claim:
A variable-magnification projection optical system substantially consisting of, in order from an enlargement side: a first lens group having a positive refractive power and being fixed during magnification change; a second lens group having a negative refractive power and being moved during magnification change; a third lens group having a positive refractive power and being moved during magnification change; a fourth lens group having a positive refractive power and being moved during magnification change; and a fifth lens group having a positive refractive power and being fixed during magnification change, wherein the variable-magnification projection optical system is configured to be telecentric at a reduction side, and conditional expressions (1) and (2) below are satisfied: L/Imφ< 15.0 (1), and 1.3<Bf/ fw< 3.0 (2), where L is a distance along an optical axis from a most enlargement-side lens surface to a most reduction-side lens surface when a projection distance is infinity, Imφ is a maximum effective image circle diameter at the reduction side, Bf is a back focus, which is an equivalent air distance, of the entire system at a wide-angle end, and fw is a focal length of the entire system at the wide-angle end.