Variable-magnification observation optical system

A variable-magnification observation optical system comprising:

• an objective system;
• an erecting system that erects an inverted image formed by the objective system;
• and
• an eyepiece system that allows the erect image formed by the erecting system to be observed through a pupil,
• wherein the objective system includes a first group having positive power, a second group having positive power, and a third group having negative power in order from an object side,
• wherein the eyepiece system includes a fourth group having positive power and a fifth group having positive power in order from the object side,
• wherein the erecting system is located between the first group and the second group,
• wherein zooming from a low magnification end to a large magnification end involves moving the third group and the fourth group in opposite directions along an optical axis while locating an image plane between the third group and the fourth group,
• wherein the fifth group includes, in order from the object side, a negative meniscus lens and a positive lens with an air gap therebetween, the negative meniscus lens having a concave surface facing the object, and
• wherein the fifth group has at least one aspheric surface
• which satisfies the following conditional equation (1):
• 0.2<LT5/few<0.3 (1)
• wherein:
• LT5 is a distance on an optical axis between a pupil-side surface of a lens 5-1 and an object-side surface of a lens 5-2,
• provided that the negative meniscus lens in the fifth group is defined as the lens 5-1 and the positive lens in the fifth group as the lens 5-2, and
• few is a focal length of the eyepiece system at the low magnification end.

A variable-magnification observation optical system comprising:

• an objective system;
• an erecting system that erects an inverted image formed by the objective system;
• and
• an eyepiece system that allows the erect image formed by the erecting system to be observed through a pupil,
• wherein the objective system includes a first group having positive power, a second group having positive power, and a third group having negative power in order from an object side,
• wherein the eyepiece system includes a fourth group having positive power and a fifth group having positive power in order from the object side,
• wherein the erecting system is located between the first group and the second group,
• wherein zooming from a low magnification end to a large magnification end involves moving the third group and the fourth group in opposite directions along an optical axis while locating an image plane between the third group and the fourth group,
• wherein the fifth group includes, in order from the object side, a negative meniscus lens and a positive lens with an air gap therebetween, the negative meniscus lens having a concave surface facing the object, and
• wherein the fifth group has at least one aspheric surface,
• which satisfies the following conditional equation (2):
• 0.5<f4/few<0.8 (2)
• wherein:
• f4 is a focal length of the fourth group, and
• few is a focal length of the eyepiece system at the low magnification end.

A variable-magnification observation optical system comprising:

• an objective system;
• an erecting system that erects an inverted image formed by the objective system;
• and
• an eyepiece system that allows the erect image formed by the erecting system to be observed through a pupil,
• wherein the objective system includes a first group having positive power, a second group having positive power, and a third group having negative power in order from an object side,
• wherein the eyepiece system includes a fourth group having positive power and a fifth group having positive power in order from the object side,
• wherein the erecting system is located between the first group and the second group,
• wherein zooming from a low magnification end to a large magnification end involves moving the third group and the fourth group in opposite directions along an optical axis while locating an image plane between the third group and the fourth group,
• wherein the fifth group includes, in order from the object side, a negative meniscus lens and a positive lens with an air gap therebetween, the negative meniscus lens having a concave surface facing the object, and
• wherein the fifth group has at least one aspheric surface,
• which satisfies the following conditional equation (4):
• 0.4<f34t/f34w<0.8 (4)
• wherein:
• f34w is a composite focal length of the third group and the fourth group at the low magnification end, and
• f34t is a composite focal length of the third group and the fourth group at the high magnification end.