Patent ID: 8373933

Claim:
An optical system comprising, in order from a magnifying-conjugate side, a first lens unit having a negative refractive power, and a rear group having one of a positive refractive power and a negative refractive power and including at least one lens unit, wherein each interval between lens units including the first lens unit and the at least one lens unit of the rear group is changed during zooming, the optical system further comprising: an aperture stop; and a first positive lens provided in the reduction-conjugate side of the aperture stop, wherein the first lens unit is provided in the most magnifying-conjugate side, wherein the following conditions are satisfied: 0.802≦θ gFz −(1 .000×10 −4 ×V dz 2 −9.10×10 −3 ×v dz )<0.86, 10<v dz <30, and 0.5 <f Gz |√{square root over ( f W ×f T )}<1.9 where f Gz indicates a focal length of the first positive lens, f W indicates a focal length of the entire optical system at a wide angle end, f T indicates a focal length of the entire optical system at a telephoto end, N gz , N Fz , N dz , and N Cz indicate refractive indices of the first positive lens with respect to a g-line, an F-line, a d-line, and a C-line, respectively, and θ gFz and v dz indicate a partial dispersion ratio and an Abbe number of the first positive lens, respectively, and where v dz and θ gFz are expressed as follows: v dz =( N dz −1)/( N Fz −N Cz ), θ gFz =( N gz −N Fz )/( N Fz −N cz ), and wherein the optical system includes a first negative lens which has a minimum Abbe number with respect to the d-line among negative lenses located in the reduction-conjugate side of the aperture stop; and the first negative lens satisfies the following condition: 0 <  v Nz ⁢ ⁢ min × f Nz f W × f T  < 25 where v Nzmin indicates the Abbe number of the first negative lens and f Nz indicates a focal length of the first negative lens.