Imaging optical system, image capture device, and camera system

An imaging optical system consisting of:

• a first lens group having positive power;
• a second lens group having power;
• and
• a third lens group having power,
• the first lens group, the second lens group, and the third lens group being arranged in this order such that the first lens group is located closer to an object than any other lens group of the imaging optical system is,
• the first lens group and the third lens group being immobile along an optical axis with the second lens group moving along the optical axis, while the imaging optical system is focusing to make a transition from an infinity in-focus state toward a close-object in-focus state,
• the first lens group consisting of:
• a sub-lens group G1A;
• an aperture stop;
• and
• a sub-lens group G1B,
• the sub-lens group G1A, the aperture stop, and the sub-lens group G1B being arranged in this order such that the sub-lens group G1A is located closer to the object than the aperture stop or the sub-lens group G1B is,
• the sub-lens group G1A including, as a plurality of lenses:
• a meniscus lens L1A1 having positive power, having a convex surface facing the object, and disposed closest to the object in the plurality of lenses;
• a meniscus lens L1A2 having negative power, having a convex surface facing the object, and disposed second closest to the object in the plurality of lenses;
• and
• a meniscus lens L1A3 having negative power, having a convex surface facing the object, and disposed third closest to the object in the plurality of lenses,
• the imaging optical system satisfying the following Inequality (1):
• 0.3<BF/Y<1.2 (1)
• where BF is a back focus of the imaging optical system, and
• Y is an image height of the imaging optical system in the infinity in-focus state.