A lens assembly includes a first lens group, a second lens group, a third lens group, a fourth lens group, and a reflective element. The first lens group is with refractive power. The second lens group is with positive refractive power. The third lens group is with positive refractive power. The fourth lens group is with refractive power. The reflective element includes a reflective surface. A light from an object sequentially passes through the first lens group, the second lens group, the third lens group, and the fourth lens group to an image side along an axis. The reflective element is disposed between an object side and the image side along the axis. Intervals of the lens groups are changeable when the lens assembly zooms from a wide-angle end to a telephoto end.

A lens barrel which prevents an increase in driving force for optical units in a direction of an optical axis to downsize a power transmission mechanism, save power and also prevents undesired movement of the optical units even when they are urged by urging forces large enough to prevent rattling. The optical units move in the direction of the optical axis, and they are urged by urging member in the direction of the optical axis. A camshaft provided on outer peripheries of the optical units extends in the direction of the optical axis and is cam-engaged with each of the optical units. The power transmission mechanism transmits driving force, which is for rotating the camshaft, to the camshaft. A torque generated in the camshaft by the urging member urging one of the optical units and another torque generated in the camshaft by the urging member urging another one of the optical units are opposite in direction.

An optical system used in an imaging apparatus includes an optical element. In a case where the optical element is disposed on a magnification side with respect to an intersection point of an optical axis and a chief paraxial ray, the optical element is a negative lens, and in a case where the optical element is disposed on a reduction side with respect to the intersection point, the optical element is a positive lens. The optical element is configured to satisfy all of the following conditional expressions:

30d40

1.225[nd(14.387/d)]1.276

0.4300[gF(2.9795/d)]0.5010,

where d represents an Abbe number of the optical element, gF is a partial dispersion ratio of the optical element for g-line and F-line, and nd is a refractive index of the optical element for d-line.

An optical system includes an optical element. When the optical element is disposed on a magnification side with respect to an intersection point between an optical axis and a chief paraxial ray, the optical element is a positive lens. When the optical element is disposed on a reduction side with respect to the intersection point, the optical element is a negative lens. The optical element satisfies all of the following conditional expressions:

30d40

1.225[nd(14.387/d)]1.276

0.4300[gF(2.9795/d)]0.5010

where d is Abbe number of the optical element, gF is a partial dispersion ratio of the optical element for g-line and F-line, and nd is a refractive index of the optical element for d-line.

An interchangeable lens attachable to a camera body, the interchangeable lens including: a focusing optical system configured to vary a focal position of the interchangeable lens; and a transmitter configured to transmit a first value and a second value to the camera body, the first value indicating a relationship between an amount of movement of the focusing optical system and an amount of movement of an image plane at a position to which the focusing optical system has moved, the second value indicating a relationship between an amount of movement of the focusing optical system and an amount of movement of the image plane, the second value being equal to or smaller than the first value.

A zoom lens forms an intermediate image at a position conjugate to a reduction side imaging plane and forms the intermediate image again on a magnification side imaging plane. The zoom lens includes a plurality of lens groups including at least two movable lens groups, which move by changing spacings between the groups adjacent to each other in a direction of an optical axis during zooming, at a position closer to the reduction side than the intermediate image. Among the plurality of lens groups, a final lens group closest to the reduction side has a positive refractive power, and remains stationary with respect to the reduction side imaging plane during zooming. The zoom lens satisfies predetermined conditional expressions (1) and (2) about the focal lengths of the movable lens groups.

A zoom lens forms an intermediate image at a position conjugate to a reduction side imaging plane and forms the intermediate image again on a magnification side imaging plane. The zoom lens includes a plurality of lens groups including at least two movable lens groups, which move by changing spacings between the groups adjacent to each other in a direction of an optical axis during zooming, at a position closer to the reduction side than the intermediate image. Among the plurality of lens groups, a final lens group closest to the reduction side has a positive refractive power, and remains stationary with respect to the reduction side imaging plane during zooming. The zoom lens satisfies predetermined conditional expressions (1) and (2).

The zoom lens is a lens system that forms an intermediate image, and consists of, in order from the magnification side: a first lens group that remains stationary during zooming; a plurality of movable lens groups that move during zooming; and a final lens group that has a positive power and remains stationary during zooming. Two or more movable lens groups are positioned to be closer to the reduction side than the intermediate image. The lens system closer to the reduction side than the intermediate image consists of, in order from the magnification side, a front group and a rear group. The zoom lens satisfies predetermined conditional expressions (1) and (2) relating to the rear group.

A projection optical device includes a fourth lens group movable along an optical axis, a fifth lens group provided in a position maintained within the projection optical device, a fourth lens frame holding the fourth lens group and having cam pins, a guide tube having a rectilinear groove, the cam pins inserted into the rectilinear groove, and a cam tube rotated with respect to the guide tube to guide the cam pins with the rectilinear groove and the cam groove, and moving the fourth lens frame along the optical axis, wherein the cam pins are formed on an upstream side in an optical path of the fourth lens group in a direction along the optical axis, and the fifth lens group is provided on the upstream side in the optical path of the fourth lens group.

The invention relates to an electronic image pickup system whose depth dimension is extremely reduced, taking advantage of an optical system type that can overcome conditions imposed on the movement of a zooming movable lens group while high specifications and performance are kept. The electronic image pickup system comprises an optical path-bending zoom optical system comprising, in order from its object side, a 1-1st lens group G1-1 comprising a negative lens group and a reflecting optical element P for bending an optical path, a 1-2nd lens group G1-2 comprising one positive lens and a second lens group G2 having positive refracting power. For zooming from the wide-angle end to the telephoto end, the second lens group G2 moves only toward the object side. The electronic image pickup system also comprises an electronic image pickup device I located on the image side of the zoom optical system.