Patent ID: 8467125

Claim:
An image taking optical system comprising at least one lens provided on an optical path; a first infrared-ray absorption filter made from a resin material with a film shape and provided on said optical path; a multi-layer film provided with an infrared-ray absorption function and provided on said optical path; and a second infrared-ray absorption filter made from a resin material with a film shape and provided on said optical path, wherein said first infrared-ray absorption filter, said multi-layer film and said second infrared-ray absorption filter are provided at locations arranged along said optical path in a direction from a photographing-object side to an image side, and said multi-layer film has a spectroscopic-characteristic adjustment function and a light reflection characteristic, said image taking optical system satisfying conditions (1) to (4) given as follows: 0.84 <R 1 /R 2<1.2 (1) 0.8<λ1/λ2<1.25 (2) T 2 /T 1<1.0 (3) T 3 /T 4<0.05 (4) where reference notation R1 denotes an average reflectance exhibited by said multi-layer film for light having a wavelength in the range 680 nm to 780 nm, passing through said first infrared-ray absorption filter from said photographing-object side to be reflected by said multi-layer film and passing through said first infrared-ray absorption filter from said image side, reference notation R2 denotes an average reflectance exhibited by said multi-layer film for light having a wavelength in said range 680 nm to 780 nm, passing through said second infrared-ray absorption filter from said image side to be reflected by said multi-layer film and passing through said second infrared-ray absorption filter from said photographing-object side, reference notation λ1 denotes the value of a wavelength for which a transmittance exhibited by said first infrared-ray absorption filter for light having a wavelength at least equal to 550 nm becomes equal to 70%, reference notation λ2 denotes the value of a wavelength for which a transmittance exhibited by said second infrared-ray absorption filter for light having a wavelength at least equal to 550 nm becomes equal to 70%, reference notation T1 denotes an average transmittance exhibited by a combination of said first infrared-ray absorption filter and said second infrared-ray absorption filter for light having a wavelength in the range 700 nm to 725 nm, reference notation T2 denotes an average transmittance exhibited by said multi-layer film for light having a wavelength in said range 700 nm to 725 nm, reference notation T3 denotes a transmittance exhibited by a combination of said first infrared-ray absorption filter, said multi-layer film and said second infrared-ray absorption filter for light having a wavelength of 700 nm, and reference notation T4 denotes a transmittance exhibited by said combination of said first infrared-ray absorption filter, said multi-layer film and said second infrared-ray absorption filter for light having a wavelength of 540 nm.