Provided is an image pickup optical system capable of performing focusing
in a wide region without a large increase in tilt angle in the case where
an image of an object is picked up from a low angle. The image pickup
optical system includes a first optical system and a second optical
system, for enlarging an image formed by the first optical system and
forming the enlarged image. A principal plane of the first optical system
is tilted relative to an image plane of the second optical system.