Observation optical system and optical apparatus

An observation optical system consisting of, in order from an object side to an eyepoint side, two lenses including an objective lens having a negative refractive power and an eyepiece lens having a positive refractive power, wherein, in a case where:

• a paraxial curvature radius of a surface of the eyepiece lens on the eyepoint side is denoted by R4,
• a paraxial curvature radius of a surface of the objective lens on the eyepoint side is denoted by R2,
• a distance from an intersection between a surface of the objective lens on the object side and an optical axis to an intersection between the surface of the eyepiece lens on the eyepoint side and the optical axis is denoted by Dsum,
• a focal length of the objective lens is denoted by f1,
• a focal length of the eyepiece lens is denoted by f2,
• an average value of refractive indices of all lenses included in the observation optical system on a d line is denoted by Nave, and
• a center thickness of the eyepiece lens is denoted by D2,
• Conditional Expressions (1), (2), (3), and (4) are satisfied:
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