An optical system consists of, 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 positive refractive power, a third lens unit having a negative refractive power. A distance between adjacent lens units changes during focusing. The second lens unit moves during the focusing. The following conditional expression is satisfied:

0.20<f3/f<0.05

where f is a focal length of the optical system focused on an object at infinity and f3 is a focal length of the third lens unit.

An optical system consists of, 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 positive refractive power, a third lens unit having a negative refractive power. A distance between adjacent lens units changes during focusing. The second lens unit moves during the focusing. The following conditional expression is satisfied:

0.20<f3/f<0.05

where f is a focal length of the optical system focused on an object at infinity and f3 is a focal length of the third lens unit.

Provided is an optical system including, 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 configured to move during focusing; and a third lens unit, wherein an interval between each pair of adjacent lens units is changed during focusing, wherein the first lens unit includes a positive lens arranged closest to the object side and an aperture stop, wherein the second lens unit consists of one negative lens, and wherein a distance on an optical axis from a lens surface closest to the object side in the optical system to an image plane, a focal length of the optical system, a refractive index of a material of the one negative lens, and an Abbe number of the material of the one negative lens are appropriately set.

An optical system includes a plurality of lenses. The distance from the object side lens surface of a lens arranged an the most object side to the image plane is longer than the focal length of the entire system. The material of a positive lens G1p arranged on the most object side among positive lenses included in the optical system, and the arrangement and material of a negative lens Gn located on the most object side among negative lenses arranged an the image side of the positive lens G1p are appropriately set.

An optical system includes a plurality of lenses. The distance from the object side lens surface of a lens arranged an the most object side to the image plane is longer than the focal length of the entire system. The material of a positive lens G1p arranged on the most object side among positive lenses included in the optical system, and the arrangement and material of a negative lens Gn located on the most object side among negative lenses arranged an the image side of the positive lens G1p are appropriately set.

An image forming lens system includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit. The first lens unit includes a front-side lens unit having a positive refractive power and a rear-side lens unit in order from the object side. The second lens unit moves at a time of focusing. The third lens unit includes a positive lens element. Each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a). The rear-side lens unit includes a negative lens element and a positive lens element. The front-side lens unit includes a diffractive lens component having a diffractive lens surface. The diffractive lens component has a positive refractive power. Following Conditional Expression (1) is satisfied:
0.7f/fLens(a)
0.015GFGR/f0.25(1).

An image forming lens system includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit. The first lens unit includes a front-side lens unit having a positive refractive power and a rear-side lens unit in order from the object side. The second lens unit moves at a time of focusing. The third lens unit includes a positive lens element. Each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a). The rear-side lens unit includes a negative lens element and a positive lens element. The front-side lens unit includes a diffractive lens component having a diffractive lens surface. The diffractive lens component has a positive refractive power. Following Conditional Expression (1) is satisfied:
0.7f/fLens(a)
0.015GFGR/f0.25(1).

Provided is an optical system including, 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 configured to move during focusing; and a third lens unit, wherein an interval between each pair of adjacent lens units is changed during focusing, wherein the first lens unit includes a positive lens arranged closest to the object side and an aperture stop, wherein the second lens unit consists of one negative lens, and wherein a distance on an optical axis from a lens surface closest to the object side in the optical system to an image plane, a focal length of the optical system, a refractive index of a material of the one negative lens, and an Abbe number of the material of the one negative lens are appropriately set.

A partial lens unit St includes one or more positive lenses that are appropriately set based on a focal length f.sub.1 of a first lens unit, a focal length f.sub.2 of a second lens unit, an overall lens length L, a focal length f of an imaging optical system of when focused on an infinite-distance object, a focal length f.sub.Pi of an i-th positive lens in an i-th position, among the positive lenses, counted in order from an object side to an image side, an Abbe number and an anomalous partial dispersion ratio difference .sub.dPi and .sub.gFPi of a material for the i-th positive lens, and the total number n of the positive lenses included in the one or more partial lens units St.

A partial lens unit St includes one or more positive lenses that are appropriately set based on a focal length f.sub.1 of a first lens unit, a focal length f.sub.2 of a second lens unit, an overall lens length L, a focal length f of an imaging optical system of when focused on an infinite-distance object, a focal length f.sub.Pi of an i-th positive lens in an i-th position, among the positive lenses, counted in order from an object side to an image side, an Abbe number and an anomalous partial dispersion ratio difference .sub.dPi and .sub.gFPi of a material for the i-th positive lens, and the total number n of the positive lenses included in the one or more partial lens units St.