Variable-magnification projection optical system and projection display device

A variable-magnification projection optical system substantially consisting of:

• a first lens group that has a negative refractive power, is disposed at a most enlargement-side position and is fixed during magnification change;
• a last lens group that has a positive refractive power, is disposed at a most reduction-side position and is fixed during magnification change;
• and a plurality of lens groups that are disposed between the first lens group and the last lens group and are moved during magnification change,
• wherein a most enlargement-side lens group of the lens groups that are moved during magnification change has a negative refractive power,
• the variable-magnification projection optical system is configured to be telecentric on a reduction side, and
• conditional expressions (1-1), (2) and (5) below are satisfied:
• 2.0<Bf/fw (1-1),
• −13.0<f2/fw<−1.5 (2) and
• 1.0<fe/fw<5.0 (5),
• where Bf is a back focus, which is an equivalent air distance, on the reduction side of the entire system at a wide-angle end, fw is a focal length of the entire system at the wide-angle end, f2 is a focal length of the most enlargement-side lens group of the lens groups that are moved during magnification change, and fe is a focal length of the last lens group.