A plurality of imaging optical systems includes at least two imaging optical systems, and each imaging optical system includes an identical diaphragm member. Each imaging optical system includes in order from an object side, a front lens unit having a positive refractive power, a focusing lens unit having a negative refractive power, and a rear lens unit having a positive refractive power. The diaphragm member is disposed near the focusing lens unit. At the time of focusing, only the focusing lens unit moves on an optical axis. Each imaging optical system satisfies the following conditional expression (1), and the plurality of imaging optical systems satisfies the following conditional expressions (2) and (3).
0.5<f.sub.ff/f.sub.fb<1.9(1)
1AP.sub.max/AP.sub.min1.15(2)
1.02<f.sub.foLA/f.sub.foSM<2.50(3)

A plurality of imaging optical systems includes at least two imaging optical systems, and each imaging optical system includes an identical diaphragm member. Each imaging optical system includes in order from an object side, a front lens unit having a positive refractive power, a focusing lens unit having a negative refractive power, and a rear lens unit having a positive refractive power. The diaphragm member is disposed near the focusing lens unit. At the time of focusing, only the focusing lens unit moves on an optical axis. Each imaging optical system satisfies the following conditional expression (1), and the plurality of imaging optical systems satisfies the following conditional expressions (2) and (3).
0.5<f.sub.ff/f.sub.fb<1.9(1)
1AP.sub.max/AP.sub.min1.15(2)
1.02<f.sub.foLA/f.sub.foSM<2.50(3)

In a converter device that is attached between a lens device, which is attachable to and detachable from an image capturing apparatus, and the image capturing apparatus, and that has a reimaging optical system configured to reimage a primary image formed by the lens device on an image plane as a secondary image, the imaging magnification of the reimaging optical system and imaging positions of a primary image and a secondary image are appropriately set.

A rear conversion lens includes: a chassis including two opposite ends, the chassis including light-transmitting holes on the ends, respectively; a first coupler configured to detachably couple one of the ends of the chassis with an image field side of an image-pickup lens of a three-chip camera including three first image sensors, the image-pickup lens being designed based on an assumption that the image-pickup lens is to be used with a color-separation prism in combination; a second coupler configured to detachably couple the other end of the chassis with a single-chip color camera including a second image sensor, the second image sensor including an image field larger than an image field of the first image sensors of the three-chip camera; and an optical system including a first lens group, a second lens group, and a third lens group arranged in the chassis in this order from an object side, the first lens group having a negative compound focal length, the second lens group being configured to correct a spherical aberration, the third lens group being configured to form an image.

A rear conversion lens includes: a chassis including two opposite ends, the chassis including light-transmitting holes on the ends, respectively; a first coupler configured to detachably couple one of the ends of the chassis with an image field side of an image-pickup lens of a three-chip camera including three first image sensors, the image-pickup lens being designed based on an assumption that the image-pickup lens is to be used with a color-separation prism in combination; a second coupler configured to detachably couple the other end of the chassis with a single-chip color camera including a second image sensor, the second image sensor including an image field larger than an image field of the first image sensors of the three-chip camera; and an optical system including a first lens group, a second lens group, and a third lens group arranged in the chassis in this order from an object side, the first lens group having a negative compound focal length, the second lens group being configured to correct a spherical aberration, the third lens group being configured to form an image.

A rear converter lens consists of three lens groups of, in order from its object-side, a first lens-group having positive refractive power; a second lens-group having negative refractive power; and a third lens-group having positive refractive power. The first lens-group consists of a cemented lens of a negative lens having a concave surface facing an image-side of the rear converter lens and a positive lens cemented together in order from the object-side. The second lens-group consists of a cemented lens of a negative lens, a lens having a biconvex shape, and a negative lens cemented together in order from the object-side. The third lens-group includes a lens having a convex surface facing the object-side furthest toward the object-side. When the focal length of the third lens-group is f3 and the focal length of the rear converter lens is cf, conditional expression (1) (1.9<f3/cf<0.4) is satisfied.

An image pickup apparatus includes: a mount to which an interchangeable lens is mountable; an image sensor; an optical system having a refractive power and imaging an object point located on an image side of a most-object-side surface of the optical system onto the image sensor; and an optical filter arranged between the optical system and the image sensor, and a distance on an optical axis from a flange surface of the mount to the object point located on the image side of the most-object-side surface, a distance on the optical axis from the flange surface of the mount to the image pickup element, and a lateral magnification of the optical system when the interchangeable lens is mounted to the image pickup apparatus and the object point located on the image side of the most-object-side surface, is imaged onto the image pickup element, are appropriately set.

An image pickup apparatus includes: a mount to which an interchangeable lens is mountable; an image sensor; an optical system having a refractive power and imaging an object point located on an image side of a most-object-side surface of the optical system onto the image sensor; and an optical filter arranged between the optical system and the image sensor, and a distance on an optical axis from a flange surface of the mount to the object point located on the image side of the most-object-side surface, a distance on the optical axis from the flange surface of the mount to the image pickup element, and a lateral magnification of the optical system when the interchangeable lens is mounted to the image pickup apparatus and the object point located on the image side of the most-object-side surface, is imaged onto the image pickup element, are appropriately set.

An optical apparatus includes a display that displays an image, and an optical system that includes a filter (a reflective polarizing plate) and a lens (a half mirror surface) arranged on a downstream side and an upstream side, respectively, on an optical axis L of a display and magnifies the image by at least the lens (half mirror surface). The optical apparatus drives the lens along the optical axis L with respect to the filter by a mobile device, or changes the surface shape or the lens power of the lens having a variable surface shape or variable lens power. Thus, the optical path is folded back twice between the filter and the lens of the optical system, and the image is magnified by the lens (the half mirror surface), so that the position of the magnified virtual image can be adjusted according to the diopter of the user.

An optical apparatus includes a display that displays an image, and an optical system that includes a filter (a reflective polarizing plate) and a lens (a half mirror surface) arranged on a downstream side and an upstream side, respectively, on an optical axis L of a display and magnifies the image by at least the lens (half mirror surface). The optical apparatus drives the lens along the optical axis L with respect to the filter by a mobile device, or changes the surface shape or the lens power of the lens having a variable surface shape or variable lens power. Thus, the optical path is folded back twice between the filter and the lens of the optical system, and the image is magnified by the lens (the half mirror surface), so that the position of the magnified virtual image can be adjusted according to the diopter of the user.