An imaging optical system includes three lens elements. The three lens elements, in order from an object side to an image side, are a first lens element having an object-side surface facing the object side and an image-side surface facing the image side, a second lens element having an object-side surface facing the object side and an image-side surface facing the image side, and a third lens element having an object-side surface facing the object side and an image-side surface facing the image side. The third lens element has positive refractive power, the imaging optical system has a total of three lens elements, and there are air gaps between paraxial regions of the first lens element, the second lens element and the third lens element.

An imaging optical system includes three lens elements. The three lens elements, in order from an object side to an image side, are a first lens element having an object-side surface facing the object side and an image-side surface facing the image side, a second lens element having an object-side surface facing the object side and an image-side surface facing the image side, and a third lens element having an object-side surface facing the object side and an image-side surface facing the image side. The third lens element has positive refractive power, the imaging optical system has a total of three lens elements, and there are air gaps between paraxial regions of the first lens element, the second lens element and the third lens element.

A rear converter lens consists of, in order from the object side, a positive first lens group, a negative second lens group, and a positive third lens group. The first lens group consists of a cemented lens in which a negative lens concave toward the image side and a positive lens convex toward the object side are cemented, the second lens group consists of a cemented lens in which a negative lens concave toward the image side, a positive lens convex toward both sides, and a negative lens concave toward the object side are cemented, and the third lens group consists of, in order from the object side, a third lens group-first lens that is a positive lens convex toward the object side and a third lens group-second lens that is a negative lens.

An optical system includes an aperture stop, and a focus lens unit arranged on an image side of the aperture stop, the focus lens unit having a negative refractive power and moving during focusing. The focus lens unit moves toward the image side during focusing from infinity to close distance. The focus lens unit includes a positive lens PL and a negative lens NL. An Abbe number dNL of the material of the negative lens NL, a refractive index NdNL of the material of the negative lens NL at d-line, and an Abbe number dPL of the material of the positive lens PL are set appropriately.

An optical system includes an aperture stop, and a focus lens unit arranged on an image side of the aperture stop, the focus lens unit having a negative refractive power and moving during focusing. The focus lens unit moves toward the image side during focusing from infinity to close distance. The focus lens unit includes a positive lens PL and a negative lens NL. An Abbe number dNL of the material of the negative lens NL, a refractive index NdNL of the material of the negative lens NL at d-line, and an Abbe number dPL of the material of the positive lens PL are set appropriately.

A shutter assembly comprises a plurality of shutter blades movable between an open position and a closed position, and a plurality of drive mechanisms arranged circumferentially about the plurality of shutter blades for moving the plurality of shutter blades, wherein each of said drive mechanisms is stable in at least a first position and a second position.

An objective includes a first lens group having positive power, a second lens group including a pair of lens components having concave surfaces facing each other, and a third lens group having positive power, and the pair of lens components is cemented lenses, and the following conditional expression is satisfied.
0.20d2/L0.5(1) where d2 is a distance between a surface situated closest to an image side in a first lens component and a surface situated closest to the object side in a second lens component, the first lens component being a lens component situated on the object side and the second lens component being a lens component situated on the image side of the pair of lens components. L is a distance between a surface situated closest to the object side and a surface situated closest to the image side in the objective.

An objective includes a first lens group having positive power, a second lens group including a pair of lens components having concave surfaces facing each other, and a third lens group having positive power, and the pair of lens components is cemented lenses, and the following conditional expression is satisfied.
0.20d2/L0.5(1) where d2 is a distance between a surface situated closest to an image side in a first lens component and a surface situated closest to the object side in a second lens component, the first lens component being a lens component situated on the object side and the second lens component being a lens component situated on the image side of the pair of lens components. L is a distance between a surface situated closest to the object side and a surface situated closest to the image side in the objective.