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,
• 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 sum of an air conversion distance on an optical axis from a display surface of the display element to a lens surface of the eyepiece lens closest to the display element side and a distance on the optical axis from the lens surface of the eyepiece lens closest to the display element side to a lens surface of the eyepiece lens closest to the eyepoint side in a state where the diopter is −1 diopter is denoted by TLA, and a refractive index of the first negative lens with respect to a d line is denoted by Nn1, Conditional Expressions (1), (2), and (3) are satisfied, which are represented by
• 0.355<H/f<0.48 (1)
• 0.17<H/TLA<0.285 (2)
• 1.5<Nn1<1.7 (3), and
• in a case where a refractive index of a lens having strongest positive refractive power among the lenses included in the eyepiece lens with respect to the d line is denoted by Nps, Conditional Expression (12) is satisfied, which is represented by
• 1.6<Nps<2 (12).