Patent ID: 7436488

Claim:
A projection optical system comprising: at least one optical element; and at least one positive lens, wherein the following expressions are satisfied: 0.02<Σ i {( Nf ) i −( Nc ) i}×Di/f< 0.15 θ p,av −(0.648−0.001682×ν p,av )>0 θ p,av −(0.589−0.000584×ν p,av )<0 where θ p ={( Ng ) p −( Nf ) p }/{( Nf ) p −( Nc ) p } θ p,av=Σ p (θ p/fp )/Σ p (1/ fp ) ν p,av=Σ p (ν p/fp )/Σ p (1/ fp ) where (Nf)i represents a refractive index of a material of an ith optical element for a F-line, the ith optical element being counted from an enlargement conjugate side in the projection optical system among the at least one optical element; (Nc)i represents a refractive index of the material of the ith optical element for a C-line; Di represents a material thickness of the ith optical element along an optical axis, wherein the value of Di is positive when a ray bundle that has passed through the ith optical element converges and wherein the value of Di is negative when the ray bundle diverges; f represents a focal length of the projection optical system; fp represent a focal length of a pth positive lens, the pth positive lens being counted from the enlargement conjugate side in the projection optical system among the at least one positive lens; νp represents an Abbe number of a material of the pth positive lens for a d-line; (Ng)p represents a refractive index of the pth positive lens for a g-line; (Nf)p represents a refractive index of the pth positive lens for the F-line; and (Nc)p represents a refractive index of the pth positive lens for the C-line.