Image pickup optical system, image pickup optical device, and digital equipment

An image pickup optical system for forming an optical image of an object onto a light-receiving surface of an image pickup element, the image pickup optical system comprising:

• two reflection prisms for bending incident light at an almost right angle, wherein when a reflection prism located at an object side along an optical path is a first prism and a reflection prism located at an image side along the optical path is a second prism, an optical axis at an incident surface of the first prism and the optical axis at an outgoing surface of the second prism are almost parallel with each other;
• and
• a lens element forming at least one power group on an optical path between the two reflection prisms, wherein a power group closest to an incident surface of the second prism is a positive power group and the incident surface of the second prism has a concaved surface shape facing the object side,
• wherein the image pickup optical system satisfies the conditional expressions (1) and (2):
• −4.2<fp2/f<−0.2 (1)
• 0.2<|f—1p/fp2|<1.5 (2)
• where fp2 is a focal length of the second prism,
• f is a focal length of an entire of the image pickup optical system, and
• f—1p is a focal length of the power group closest to the incident surface of the second prism.