An observation optical system includes a display element and an eyepiece lens disposed on an eyepoint side of the display element. The eyepiece lens includes a first lens having positive optical power, a second lens having negative optical power, and a third lens consecutively in order from closest to a display element side to the eyepoint side. In a case where a half value of a longest diameter of a display region in the display element is denoted by H, and a focal length of the eyepiece lens in a state where diopter is −1 diopter is denoted by f, the observation optical system satisfies a conditional expression represented by 0.35<H/f<0.6.

An observation optical system includes a display element and an eyepiece lens disposed on an eyepoint side of the display element. The eyepiece lens includes a first lens having positive optical power, a second lens having negative optical power, and a third lens consecutively in order from closest to a display element side to the eyepoint side. In a case where a half value of a longest diameter of a display region in the display element is denoted by H, and a focal length of the eyepiece lens in a state where diopter is −1 diopter is denoted by f, the observation optical system satisfies a conditional expression represented by 0.35<H/f<0.6.

A zoom lens system according to the present disclosure includes a plurality of lens groups each of which is made up of at least one lens element. The zoom lens system includes, in order from an object side to an image side: a first focusing lens group having a negative power; and a second focusing lens group having a positive power, wherein when zooming is performed from a wide end to a telephoto end, the first focusing lens group and the second focusing lens group move along an optical axis, when focusing is performed from an infinity focusing state to a proximity object focusing state, the first focusing lens group and the second focusing lens group move to perform the focusing, and a predetermined condition is satisfied.

Lens barrel includes first group lens frame unit, second group lens frame unit, lens frame, insertion hole, and depressions. First group lens frame unit holds lenses. Second group lens frame unit holds lens. Lens frame is disposed between first group lens frame unit and second group lens frame unit on an inner circumference side of first group lens frame unit through radial clearance. Insertion hole is provided on an outer peripheral surface of first group lens frame unit, and a jig of lens frame is inserted into insertion hole. Depressions are provided at a position accessible from an outside in a state where first group, second group lens frame units and lens frame are assembled, and fix lens frame to first group lens frame unit.

A zoom lens is provided. The zoom lens includes a first lens group, a second lens group, a third lens group and an aperture stop. The second lens group is disposed between the first lens group and the third lens group. The aperture stop is disposed between the first lens group and the second lens group. When the zoom lens zooms, the aperture stop is fixed without moving and the first lens group correspondingly moves.

A zoom lens has a four-group zoom configuration, in order from the object side to the image side, consisting of negative, positive, positive, and positive refractive power. The zoom lens carries out zooming from the wide-angle end to the telephoto end mainly by changing the distance between the third lens group and the fourth lens group. Also, the distance between the second lens group and the third lens group may be decreased in the zooming. Focusing is carried out mainly by moving the second lens group along the optical axis. Such a zoom lens is thin and small in size, and has a great imaging performance.

An endoscope magnification optical system including: a first lens group that has positive power and includes at least a negative lens with a concave surface facing an object side, a positive lens with a convex surface facing an image side, and a doublet obtained by bonding a negative lens and a positive lens; a second lens group that has negative power and includes at least a doublet obtained by bonding a negative lens and a positive lens; and a third lens group that has positive power and includes at least a positive lens and a doublet obtained by bonding a negative lens and a positive lens, is configured such that predetermined conditions are satisfied.

A zoom lens includes in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a negative refractive power, and includes an aperture stop which is disposed between the second lens unit and the fourth lens unit, and the first lens unit includes a negative lens and a plurality of positive lenses, and at the time of zooming from a wide angle end to a telephoto end, a distance between the first lens unit and the second lens unit widens, a distance between the second lens unit and the third lens unit narrows, and a distance between the third lens unit and the fourth lens unit changes, and the zoom lens satisfies predetermined conditional expressions.

A zoom lens comprising in order from an object side, a first lens unit having a negative refractive power, a second lens unit having a negative refractive power, a lens unit having a positive refractive power, and a rearmost lens unit having a negative refractive power, and the rearmost lens unit is positioned nearest to an image in the plurality of lens units, and at the time of zooming, distances between the lens units in the plurality of lens units change. Moreover, an image pickup apparatus includes the zoom lens, and an image pickup element having an image pickup surface.

A zoom lens system according to the present disclosure includes, in order from an object side to an image plane side, a first lens group having positive optical power; a second lens group having negative optical power; a third lens group having positive optical power; and a subsequent lens group including a fourth lens group, a fifth lens group, and a sixth lens group. The first lens group comprises, in order from the object side, a first lens element having negative optical power; a second lens element having positive optical power; a third lens element having positive optical power; and a fourth lens element having positive optical power. The fifth lens group in the subsequent lens group has negative optical power. The first lens group is fixed with respect to the image plane in zooming from a wide-end to a tele-end upon image shooting.