A variable magnification optical system consists of, in order from an object side to an image side, a first lens group, a second lens group, and a subsequent lens group. The variable magnification optical system satisfies a predetermined conditional expression for a partial dispersion ratio related to F line, C line, and a wavelength of 1970.09 nm, a d-line back focus of the variable magnification optical system at a telephoto end, a back focus in any one wavelength from a wavelength of 1300 nm to a wavelength of 2325.42 nm at the telephoto end, and a d-line focal length of the variable magnification optical system at the telephoto end.

A zoom optical system comprises, in order from an object: a first lens group (G1) having positive refractive power; a second lens group (G2) having negative refractive power; and a subsequent group (GR) including at least one lens group. Upon zooming, distances between the first lens group (G1) and the second lens group (G2) and between the second lens group (G2) and the subsequent group (GR) change. The subsequent group (GR) comprises a focusing group (Gfc) having negative refractive power for focusing. The first lens group (G1) comprises a 1-1st lens that has positive refractive power and is disposed closest to the object. A following conditional expression is satisfied:

0.20<f1/fR<0.85 where, f1 denotes a focal length of the first lens group, and fR denotes a focal length of a lens component, in the subsequent group, closest to an image surface.

An optical system (LS) Includes lenses (L22,L23) satisfying the following conditional expressions,

vdLZ<35.0, and

0.702<gFLZ+(0.00316dLZ),

where vdLZ: Abbe number of the lens with reference to d-line, and gFLZ: a partial dispersion ratio of the lens, wherein gFLZ is defined by the following expression,

gFLZ(ngLZnFLZ)/(nFLZnCLZ). wherein a refractive index of the lens with reference to g-line is ngLZ, a refractive index of the lens with reference to F-line is nFLZ, and a refractive index of the lens with reference to C-line is nCLZ.

An optical system includes one or more positive lenses and one or more negative lenses. The first positive lens is a positive lens disposed closest to an object among the one or more positive lenses. The first negative lens is a negative lens that is disposed on an image side of the first positive lens and disposed closest to the object among the one or more negative lenses. At least one of an object side surface and an image side surface of the first positive lens is an aspherical surface. At this time, a relationship between a total lens length of the optical system L0, a focal length of the optical system L0, and a distance Dpn on an optical axis from the image side surface of the first positive lens to an object side surface of the first negative lens is defined.

A method to design projection lenses or zoom lenses with distorted or uncorrected pupil aberration (mostly Spherical Aberration or spherochromatism). By introducing some pupil aberration, the designer has a new variable to correct for field and aperture aberrations. The result is a design that requires fewer lens count for the same performance parameters than more complex projection lenses, and is more compact. Lenses designed by the method are illustrated.

A zoom lens includes, in order from a reduction conjugate side to an enlargement conjugate side, a first optical system having a positive refractive power, and a second optical system having positive refractive power. The second optical system reimages an intermediate image formed by the first optical system. The second optical system does not move during zooming. The first optical system includes a first lens unit that moves during zooming and has a positive refractive power, a second lens unit that moves during zooming and has a negative refractive power, and a third lens unit that moves during zooming and has a positive refractive power. A distance between adjacent lens units in the first optical system changes during zooming.

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; and a third lens unit having a positive refractive power. The first lens unit does not move and the second lens unit and the third lens unit move in mutually different loci during zooming. Conditional expressions are satisfied as follows:
0.05<|f2/m2|<0.59, and
0.20<|f3/m3|<1.95,
where f2 is a focal length of the second lens unit, f3 is a focal length of the third lens unit, m2 is a moving amount of the second lens unit during zooming from a wide-angle end to a telephoto end, and m3 is a moving amount of the third lens unit during zooming from the wide-angle end to the telephoto end.

An optical system comprises a lens satisfying the following conditional expressions.

2.0100<ndLZ+(0.00925dLZ)<2.0800

28.0<dLZ<40.0 where, ndLZ: a refractive index of the lens relative to a d-line dLZ: a d-line based Abbe number of the lens.

An observation optical system including: a display element; and an eyepiece lens disposed on an eye point side of the display element, wherein the eyepiece lens consists of, in order from a display element side toward the eye point side, a first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a positive refractive power, and in a case where a longest diameter of a display area in the display element is H, and a focal length of the eyepiece lens is fA, a conditional expression (1) is satisfied:

0.7<H/fA<0.8 (1).

An ocular lens includes from an object side: a first lens group having negative refracting power, and a second lens group having positive refracting power. An object-side focal plane of the second lens group is positioned between the first lens group and the second lens group. The first lens group includes, from the object side: a meniscus first A lens component whose convex surface is directed toward the object side, and a first B lens component having negative refracting power. The second lens group includes, in a position closest to the object side, a meniscus second A lens component whose concave surface is directed toward the object side, and satisfies conditional expressions.