Patent ID: 7106530

Claim:
An optical system mountable to an image pick up lens, comprising: an image rotator which is disposed on an image side with respect to a first imaging plane of the image pick up lens and has an incident optical axis and an emission optical axis made coaxial to each other; a first lens unit having a negative optical power which is disposed between the image pick up lens and the first imaging plane; a second lens unit having a positive optical power which is disposed between the first imaging plane and the image rotator; and a third lens unit having a positive optical power which is disposed on the image side with respect to the image rotator, wherein the following conditional expressions (1) and (2) are satisfied: 0.3 <|Φ b /Φ a |<5.5 (1), −0.3<β r <−0.9 (2), where Φ a represents an optical power of the first lens unit, Φ b represents an optical power of the second lens unit, and β r represents an imaging magnification of the entire optical system.