Observation optical system and optical apparatus

An observation optical system comprising:

• a display element;
• and
• an eyepiece lens arranged on an eyepoint side of the display element,
• wherein the eyepiece lens includes at least one negative lens, and
• in a case where a half value of a longest diameter of a display region in the display element is denoted by H, and a focal length of the eyepiece lens in a state where diopter is −1 diopter is denoted by f, Conditional Expression (1) is satisfied, which is represented by
• 0.35<H/f<0.6 (1),
• wherein the eyepiece lens includes at least one positive lens on a display element side of the at least one negative lens, and
• in a case where a positive lens closest to the display element side among positive lenses included in the eyepiece lens is referred to as a display side positive lens, and a distance on an optical axis from a surface of the display side positive lens on 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 DL, Conditional Expression (14) is satisfied, which is represented by
• 0.75<DL/f<2.2 (14), and
• wherein in a case where a negative lens having strongest optical power among negative lenses included in the eyepiece lens is referred to as a first negative lens, an air interval on an optical axis between the first negative lens and a lens adjacent to the eyepoint side of the first negative lens is denoted by Dgnr, and a focal length of the first negative lens is denoted by fn, Conditional Expression (17) is satisfied, which is represented by
• −0.7<Dgnr/fn<−0.01 (17).