| 摘要 |
An image pickup optical system has an optical element of a transparent, optical material including an entrance surface, at least one reflective surface, and an exit surface. The optical element has an optical power, the image pickup optical system has an image pickup element including a plurality of pixels. On the image pickup element an image is formed by light from an object through the optical element. The optical material is a material having an index change amount DELTAn from an absolute dry condition at the temperature of 50° C. to saturation under a circumstance of the temperature being 50° C. and the humidity being 90%, the index change amount DELTAn satisfying the following condition: DELTAn<=Fdeltar02/(2f2L) where F is the F-number of the image pickup optical system, f is the focal length of the image pickup optical system, delta is the pitch of the pixels of the image pickup element, L is the optical path length of the optical element along the reference axis, and r0 is half of the length of the optical element along a direction perpendicular to a plane including the reference axis.
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