An imaging lens system includes an optical path folding member including a forwardmost reflective surface disposed closest to an object side, a rearmost reflective surface disposed closest to an imaging plane, and a rear reflective surface disposed to form an acute angle with the rearmost reflective surface and configured to reflect light reflected by the rearmost reflective surface to the imaging plane, and a first lens group disposed on an object side of the forwardmost reflective surface or an image side of the forwardmost reflective surface, wherein an angle between a first virtual plane including the forwardmost reflective surface and a second virtual plane including the rearmost reflective surface is 15 to 27 degrees.

An optical system used in an optical apparatus is configured to include a first lens group having positive refractive power, a focusing group that moves along an optical axis at focusing, and a rear group, in order from an object side, so that the first lens group includes a first-A lens group disposed on the object side of the largest air space A in the first lens group, and that all of the following conditional expressions are satisfied:

1.00<FNo×(TL/f).sup.2<2.50 and 0.30<dA/dG1<0.85

where FNo is the f-number of the optical system focusing on infinity, TL is the total optical length of the optical system focusing on infinity, f is the focal length of the optical system focusing on infinity, dA is the length on the optical axis of the air space A, and dG1 is the length on the optical axis of the first lens group.

An optical system used in an optical apparatus is configured to include a first lens group having positive refractive power, a focusing group that moves along an optical axis at focusing, and a rear group, in order from an object side, so that the first lens group includes a first-A lens group disposed on the object side of the largest air space A in the first lens group, and that all of the following conditional expressions are satisfied:

1.00<FNo×(TL/f).sup.2<2.50 and 0.30<dA/dG1<0.85

where FNo is the f-number of the optical system focusing on infinity, TL is the total optical length of the optical system focusing on infinity, f is the focal length of the optical system focusing on infinity, dA is the length on the optical axis of the air space A, and dG1 is the length on the optical axis of the first lens group.

Provided an imaging apparatus including a first optical device, a second optical device disposed such that light transmitted through the first optical device is incident on the second optical device, and a third optical device disposed such that light transmitted through the second optical device is incident on the third optical device, wherein at least one of the first optical device, the second optical device, and the third optical device includes a plurality of nanostructures, and heights of at least two nanostructures of the plurality of nanostructures are different from each other.

Provided an imaging apparatus including a first optical device, a second optical device disposed such that light transmitted through the first optical device is incident on the second optical device, and a third optical device disposed such that light transmitted through the second optical device is incident on the third optical device, wherein at least one of the first optical device, the second optical device, and the third optical device includes a plurality of nanostructures, and heights of at least two nanostructures of the plurality of nanostructures are different from each other.

The objective optical system includes in order from an object side, a first unit having a positive refractive power, a second unit having a negative refractive power, and a third unit having a positive refractive power, and focusing is carried out by moving the second unit, the first unit and the third unit are fixed at the time of focusing, and the first unit G1 includes in order from the object side, a negative lens L1, one of a positive lens and a cemented lens, and a positive lens, and the third unit includes a positive lens, and a cemented lens of a positive lens and a negative lens, and the objective optical system satisfies the following conditional expression (3)
−20<f.sub.2/f<−5  (3) where, f.sub.2 denotes a focal length of the second unit, and f denotes a focal length of the overall objective optical system in a normal observation state.

The objective optical system includes in order from an object side, a first unit having a positive refractive power, a second unit having a negative refractive power, and a third unit having a positive refractive power, and focusing is carried out by moving the second unit, the first unit and the third unit are fixed at the time of focusing, and the first unit G1 includes in order from the object side, a negative lens L1, one of a positive lens and a cemented lens, and a positive lens, and the third unit includes a positive lens, and a cemented lens of a positive lens and a negative lens, and the objective optical system satisfies the following conditional expression (3)
−20<f.sub.2/f<−5  (3) where, f.sub.2 denotes a focal length of the second unit, and f denotes a focal length of the overall objective optical system in a normal observation state.

The optical system of the present invention includes, in order from an object side: a first lens group G1 having positive refractive power; a second lens group G2 having negative refractive power; and a third lens group G3 having positive refractive power, wherein the first lens group G1 and the third lens group G3 are fixed in an optical axis direction, while the second lens group G2 is moved in the optical axis direction to focus on from an object at infinity to an object at a finite distance, the third lens group G3 includes, in order from the object side: an object-side group G3a; an aperture stop S; and an image-side group G3b, and a specified condition is satisfied.

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 in order from 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 lens unit having a negative refractive power. The second lens unit is a focusing lens unit. At the time of zooming from a wide angle end to a telephoto end, the third lens unit and the lens unit having a negative refractive power positioned on an object side of the third lens unit move, and the following conditional expression (1) is satisfied:

φ.sub.L1<φ.sub.3GL1   (1).

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 in order from 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 lens unit having a negative refractive power. The second lens unit is a focusing lens unit. At the time of zooming from a wide angle end to a telephoto end, the third lens unit and the lens unit having a negative refractive power positioned on an object side of the third lens unit move, and the following conditional expression (1) is satisfied:

φ.sub.L1<φ.sub.3GL1   (1).