Observation optical system and optical apparatus

An observation optical system comprising:

• a display element;
• and
• an eyepiece lens disposed on an eyepoint side with respect to the display element,
• wherein the eyepiece lens includes a first negative lens having strongest negative refractive power among lenses included in the eyepiece lens, a first positive lens disposed adjacent to a display element side of the first negative lens, and a second positive lens disposed closest to the display element side among positive lenses disposed on the eyepoint side with respect to the first negative lens,
• the number of lenses included in the eyepiece lens is greater than or equal to five, and
• in a case where a half value of a longest diameter of a display region in the display element is denoted by H, a focal length of the eyepiece lens in a state where diopter is −1 diopter is denoted by f, a focal length of the first negative lens is denoted by fn1, a focal length of the second positive lens is denoted by fp2, and an average value of refractive indexes of all lenses included in the eyepiece lens with respect to the d line is denoted by Nave, Conditional Expressions (1), (6-3), (7-3), and (14-3) are satisfied, which are represented by
• 0.355
• <
• H
• /
• f
• <
• 0.48
• ,
• (
• 1
• )
• -
• 1.197
• ≤
• f
• /
• fn
• ⁢
• 1
• <
• -
• 0.3
• ,
• (
• 6
• -
• 3
• )
• 0
• <
• f
• /
• fp
• ⁢
• 2
• ≤
• 0.362
• ,
• and
• (
• 7
• -
• 3
• )
• 1.57
• <
• Nave
• <
• 1.75
• .
• (
• 14
• -
• 3
• )