Patent ID: 8379329

Claim:
An imaging optical system, comprising: an aperture stop; a first lens group disposed on an object side of the aperture stop; a second lens group disposed on an image side of the aperture stop; and a plate-like neutral density filter disposed close to a position of the aperture stop in a direction of an optical axis of the first and second lens groups, wherein the second lens group includes positive refracting power as a whole, the neutral density filter is configured to be retractable from the optical axis, and to attenuate a passing light amount, each wavelength of which light wavelengths from 400 nm to 700 nm are divided at substantially equal intervals is defined as λi (i=1-n, where n≧5), the neutral density filter satisfies Condition (1) R 1/ R 2>2.0 Condition (1), R 1 is an average reflectance on an object side face of the neutral density filter calculated according to Equation (1) R 1=[Σ{( R 11 i+R 12 i/I 1 i}]/n ( i= 1˜n) Equation (1), I 1 i is an amount of light incident upon an object side face of the neutral density filter having the wavelength λi, R 11 i is a portion of the amount of light incident upon the object side face of the neutral density filter (I 1 i ) that is reflected from an image side face of the neutral density filter, R 12 i is a portion of the amount of light incident upon the object side face of the neutral density filter (I 1 i ) that is reflected from the object side face of the neutral density filter, R 2 is an average reflectance on an image side of the neutral density filter calculated according to Equation (2) R 2=[Σ{( R 21 i+R 22 i )/ I 2 i}]/n ( i= 1˜n) Equation (2), I 2 i is an amount of light incident upon the image side face of the neutral density filter having the wavelength λi, R 21 i is a portion of the amount of light incident upon the image side face of the neutral density filter (I 2 i ) that is reflected from the object side face of the neutral density filter, and R 22 i is a portion of the amount of light incident upon the image side face of the neutral density filter (I 2 i ) that is reflected from the image side face of the neutral density filter.