Provided is a zoom lens, including, in order from an object side, a positive first lens unit, a negative second lens unit, a positive third lens unit, a negative fourth lens unit, and a positive fifth lens unit, in which an interval between each pair of adjacent lens units is changed during zooming, in which the second lens unit moves during focusing, and includes, in order from the object side to the image side, a negative lens, a negative lens, a positive lens, and a negative lens, and in which a focal length of the second lens unit, a thickness of the second lens unit on an optical axis, a refractive index of a material of the positive lens included in the second lens unit, and an average value of refractive indices of materials of the negative lenses included in the second lens unit are each appropriately set.

A zoom image pickup apparatus includes a mount portion, a zoom lens which forms an image of light incident from the mount portion, and an image pickup element which is disposed at an image forming position, wherein the zoom lens includes in order from an object 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, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power, and the fourth lens unit is a focusing lens unit, and at the time of zooming from a wide angle end to a telephoto end, only the second lens unit and the fourth lens unit move, and the following conditional expression (1) is satisfied:

φ.sub.L1<φ.sub.3GL1  (1).

On embodiment of the invention discloses an optical lens including five lens group and an aperture stop. Each lens group includes at least one lens with refractive power, and the aperture stop is fixed in a position between a second lens group and a third lens group. Further, a distance between a first lens group and a fifth lens group remains fixed during zooming or focusing, and respective distances of the second lens group and a fourth lens group relative to the fifth lens group vary during zooming or focusing.

There is provided a zoom lens including first and second lens groups independently movable in an optical axis direction in an optical system. Before and after switching between two modes different in minimum focusing distance, a lens group to rule focusing changes between the first and second lens groups or a relative position between the first and second lens groups changes.

A zoom lens consists of a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, a fourth lens group having negative refractive power, and a fifth lens group having positive refractive power in this order from an object side. The third lens group includes a cemented lens closest to the object side and a cemented lens closest to an image side, and the fourth lens group consists of a negative lens and a positive lens in this order from the object side. Distances between the lens groups change, and the fifth lens group is fixed, and at least the first lens group and the fourth lens group move when magnification is changed from the wide angle end to the telephoto end.

Provided is an optical system, which retains enhanced optical performance at any object distance, and has compact and light-weight focus lens units. Provided is an optical system including a plurality of lens units, in which an interval between adjacent lens units changes during focusing, the optical system including: a positive lens unit arranged closest to an object side; first and second focus lens units having negative refractive powers configured to move during focusing; and an intermediate lens unit having a positive refractive power and arranged between the first and second focus lens units. The first and second focus lens units are configured to move toward an image side during focusing.

A zoom image pickup apparatus includes a mount portion, a zoom lens which forms an image of light incident from the mount portion, and an image pickup element which is disposed at an image forming position. The zoom lens includes a plurality of lens unit, and a focusing lens unit, and the following conditional expressions (1) and (2) are satisfied:

1.5<f.sub.Lt/f.sub.Lw   (1), and

|(y.sub.w7d′−y.sub.w7d)/y.sub.w7d×100<0.66   (2).

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 system includes a positive first lens group, a negative second lens group, in that order from the object side, and a positive n.sup.th lens group provided closest to the image side. During zooming from the short to long focal length extremities, the first lens group and the n.sup.th lens group remain stationary and the second lens group moves. A positive m.sup.th lens group is provided between the second and n.sup.th lens groups. The following conditions (1) and (2) are satisfied:
1.23<f1/fn<1.50  (1), and
1.20<fn/fm<1.60  (2),
wherein f1, fn and fm designate the focal lengths of the first, n.sup.th and m.sup.th lens groups, respectively, wherein n is a positive integer of 4 or greater, and m is a positive integer of 3 or greater.

An image quality is improved although a medical stereomicroscope optical system and a medical observation apparatus are small and light. An objective optical system and a plurality of imaging optical systems are arranged in an order from an object side to an image side, and the imaging optical system has at least a single aspheric surface. Accordingly, a spherical aberration and a field curvature are improved, and the image quality is improved although the medical stereomicroscope optical system and a medical observation apparatus are small and light.