Patent ID: 8670193

Claim:
An optical system comprising: a negative lens unit including one or more negative lenses located closest to an object side; and a positive lens unit having positive refractive power including a plurality of lenses on an image side of the negative lens unit, wherein, when a portion of lenses among the plurality of the lenses constituting the positive lens unit is denoted by lenses LAj where j is an integer equal to or greater than 1 in order from the object side, and an anomalous partial dispersion ratio of a material of the lens LAj is denoted by ΔθgFAj, the following condition is satisfied: Δθ gFAj> 0.0025, and wherein, when the one or more negative lenses are denoted by negative lenses LNi where i is an integer equal to or greater than 1 in order from the object side, a focal length of the negative lens LNi and a refractive index of a material thereof are denoted by fni and Nni, respectively, a focal length of the lens LAj is denoted by fAj, a refractive index, Abbe number, and partial dispersion ratio of a material of the lens LAj are denoted by NAj, νdAj, and θgFAj, respectively, a focal length of the entire optical system is denoted by f, and a full aperture F-number during focusing on an infinitely distant object is denoted by Fno, and when following relational equations are set: kj= 0.07( fAj> 0) kj=− 0.05( fAj< 0) Δθ gFAj=θgFAj −(−0.0023 ·νdAj+ 0.6641)( fAj> 0) Δθ gFAj =(−0.0025 ·νdAj+ 0.6777)−θ gFAj ( fAj< 0), where kj denotes a rate of change in refractive index, the following conditions are satisfied: Σ|(Δθ gFAj/νdAj )·( f/fAj )/ Fno|> 0.001 −0.3 <f·{Σkj /( NAj·fAj )+Σ1/( Nni·fni )}<0.2.