Imaging optical system having bending optical element

1. An imaging optical system, comprising:

a bending optical element which bends a light bundle emanating from an object;

a post-bending lens system arranged on a post-bending optical axis defined by an optical axis of said imaging optical system being bent by said bending optical element; and

an image sensor, on which an image of said object is formed via said post-bending lens system;

wherein an effective optical surface of a large-diameter lens element, having an axial light bundle effective radius that is greatest among those of all lens elements of said post-bending lens system, is configured into a non-circular shape by a length of said effective optical surface from said post-bending optical axis toward a side opposite from the object side being smaller than said axial light bundle effective radius, with reference to said axial light bundle effective radius lying on a plane which includes said post-bending optical axis and extends orthogonal to a plane including both said post-bending optical axis and a pre-bending optical axis of said imaging optical system,

wherein said non-circular shape of said effective optical surface satisfies the following conditions:

RU>RL,

( RU+RL )<2 RH , and

0.5<{( RU+RL )/2 RH }<0.09,

wherein when viewed in a direction along said post-bending optical axis, where an upper direction designates a direction parallel to a pre-bending optical axis and travelling toward the object side, a lower direction designates a direction opposite to said upper direction, and a horizontal direction designates a direction that is orthogonal to a plane on which both said pre-bending optical axis and said post-bending optical axis lie,

wherein RU designates a distance between said post-bending optical axis and an outermost point of said effective optical surface of said large-diameter lens element in said upper direction within the axial light bundle effective radius,

wherein RL designates a distance between said post-bending optical axis and an outermost point of said effective optical surface of said large-diameter lens element in said lower direction within said axial light bundle effective radius, and

wherein RH designates a distance between said post-bending optical axis and an outermost point of said effective optical surface of said large-diameter lens element in said horizontal direction within said axial light bundle effective radius.

2. The imaging optical system according to claim 1 , wherein said axial light bundle effective radius that is greatest among those of all lens elements of said post-bending lens system is determined with reference to said axial light bundle effective radius lying on a plane which includes said post-bending optical axis and extends orthogonal to a plane including both said post-bending optical axis and a pre-bending optical axis of said imaging optical system.

3. The imaging optical system according to claim 1 , wherein said effective optical surface of said large-diameter lens element is circular in shape except in said lower direction, and

wherein, in said lower direction, said effective optical surface of said large-diameter lens element comprises a straight side, which is parallel to said post-bending optical axis and orthogonal to a plane on which both said post-bending optical axis and said pre-bending optical axis lie, to define a D-cut shape.

4. The imaging optical system according to claim 1 , further comprising a first lens element on said object side of said bending optical element.

5. The imaging optical system according to claim 1 , wherein said imaging optical system is a retrofocus type in which a negative lens group and a positive lens group are arranged, as a whole, in that order from said object side, and

wherein said large-diameter lens element is included in said positive lens group.

6. The imaging optical system according to claim 1 , wherein said large-diameter lens element is at a position at which an entrance pupil of said imaging optical system is similar in shape to said effective optical surface of said large-diameter lens element.

7. An imaging optical system, comprising:

a bending optical element which bends a light bundle emanating from an object;

a post-bending lens system on a post-bending optical axis defined by an optical axis of said imaging optical system being bent by said bending optical element; and

an image sensor, on which an image of said object is formed via said post-bending lens system,

wherein an outer shape of an effective optical surface of a large-diameter lens element, an axial light bundle effective radius of which is greatest among all lens elements of said post-bending lens system, is formed to be non-circular as viewed in a direction of said post-bending optical axis, and

wherein a shape of an entrance pupil of said imaging optical system satisfies the following conditions:

RU′>RL′,

( RU′+RL′ )<2 RH′ , and

0.5<{( RU′+RL′ )/2 RH′ }<0.09,

wherein, when the entrance pupil is viewed in a direction along a pre-bending optical axis, where an lower direction designates a direction parallel to said post-bending optical axis which travels toward the image sensor side, a upper direction designates a direction opposite to said lower direction, and a horizontal direction designates a direction orthogonal to a plane on which both a pre-bending optical axis and said post-bending optical axis lie,

RU′ designates a distance between said pre-bending optical axis and an outermost point of the entrance pupil in said upper direction,

RL′ designates a distance between said pre-bending optical axis and an outermost point of the entrance pupil in said lower direction, and

RH′ designates a distance between said pre-bending optical axis and an outermost point of the entrance pupil in said horizontal direction.