Patent ID: 8203793

Claim:
An optical system having a larger total lens length than a focal length and including an aperture stop in an optical path, the optical system comprising: lenses L and NL on at least one of a magnification conjugate side and a reduction conjugate side with respect to the aperture stop, wherein the lens L satisfies conditional expressions as follows, −1.68×10 −3 ·ν L +0.590<θ L <3.15×10 −4 ·ν L 2 −1.86×10 −2 ·ν L +0.878, and 5<ν L <27, where ν L and θ L are an Abbe number and a gF-line partial dispersion ratio of a material of the lens L, wherein the lens NL has a refractive power with a sign opposite to a sign of the lens L and is made of a material having a highest gF-line partial dispersion ratio, wherein the lens L has a positive refractive power when the lens L is located at the magnification conjugate side with respect to the aperture stop, or has a negative refractive power when the lens L is located at the reduction conjugate side with respect to the aperture stop, and wherein the lenses L and NL satisfy a conditional expression as follows, −0.20<(θ NL −θ L )·√{square root over (( fw·ft ))}/ f N ≦−0.03, where f N is a focal length of one of the lenses L and NL with a negative refractive power, θ NL is a gF-line partial dispersion ratio of the material of the lens NL, fw is a focal length of the optical system at the wide-angle end, and ft is a focal length of the optical system at the telephoto end, fw and ft being equivalent to f if the optical system has a single focal length f.