Patent ID: 8144237

Claim:
An image pickup optical system comprising: a first prism including: a first incident surface on which a subject light beam from a subject is incident; a division surface for dividing the subject light beam, which entered through the first incident surface, into an image pickup light beam traveling toward an image pickup element and a light beam for focus detection traveling toward an image pickup element for focus detection; and a first exit surface from which the light beam for focus detection having been reflected on the division surface is emerged; and a second prism including: a second incident surface on which the image pickup light beam having passed through the division surface is incident; and a second exit surface from which the image pickup light beam is emerged, wherein the light beam for focus detection is reflected on the division surface for a first time and then reflected on the first incident surface, and reflected on the division surface for a second time to reach the first exit surface, and wherein the image pickup optical system satisfies the following conditional expression 0.1 <D A /D B <0.5, where D A indicates an effective light beam diameter of a light beam guided to the image pickup element for focus detection, and D B indicates an effective light beam diameter of a light beam guided to the image pickup element for image pickup.