Patent ID: 8068289

Claim:
An optical system comprising, in order from an enlargement conjugate side to a reduction conjugate side: a first lens unit having a negative refractive power; and a second lens unit having a positive refractive power, wherein an aperture stop is located closer to the reduction conjugate side than the first lens unit, wherein one or more positive lenses are located closer to the reduction conjugate side than the aperture stop, wherein the one or more positive lenses include a P-th positive lens located closest to the reduction conjugate side, and wherein a relative partial dispersion of a material of the P-th positive lens (θ gFP ), an Abbe number of the material of the P-th positive lens (ν dp ), a refractive index, with respect to d-line light, of the material of the P-th positive lens (n dp ), a distance from the aperture stop to a surface of the P-th positive lens on the reduction conjugate side (d SP-P ) a distance from the aperture stop to a surface on the reduction conjugate side of a lens located closest to the reduction conjugate side (d SP-PR ), a focal length of the P-th positive lens (f P ), and a focal length of the entire optical system (f S ) satisfy the following conditions: 0.79<θ gFP −(1.00×10 −4 ×ν dp 2 −9.10×10 −3 ×ν dp )<0.86 10<ν dp <30 1.7<n dp <2.2 0.7 <d SP-P /d SP-PR ≦1.0 0.5 <f P /f S <2.2.