| 摘要 |
PROBLEM TO BE SOLVED: To provide an image forming optical system for a far infrared ray having excellent image forming performance, regardless of a wide angle, an imaging optical device including the image forming optical system for the far infrared ray, and digital equipment.SOLUTION: An image forming optical system for a far infrared ray LN comprises, in order from an object side: a first lens L1 having positive power; and a second lens L2 having positive power. The first lens L1 is a double-sided aspherical lens which is paraxial and has a positive meniscus shape convex to the object side and the second lens L2 is a double-sided aspherical lens which is paraxial and has a biconvex shape. The image forming optical system LN has a position where the vertical positions of on-axis light and ambient light are replaced between the first lens L1 and the second lens L2, when drawing a figure in the meridian plane of the entire system and satisfies a conditional expression of 0.6<(TLopt-LB)/TLopt<0.95(TLopt=TL+D1×(n1-1)+D2×(n2-1), where TL is the total length, D1 and D2 are the center thicknesses of the first and the second lenses, n1 and n2 are the refractive indexes at the reference wavelength of the first and second lenses, and LB is a distance from the final surface of an optical system to a paraxial image surface. |
