Ocular optical system and head mounted display

An ocular optical system comprising:

• a Fresnel lens including a plurality of Fresnel zones formed on a lens surface on an observation object side, wherein
• the plurality of Fresnel zones are arranged concentrically side by side along an aspherical surface having a shape which is rotationally symmetric with respect to a central axis of the Fresnel lens, and satisfy the following conditional expression:
• 0<PAE1/PAC1≤0.50,
• where
• PAE1 represents an average pitch in a radial direction of Fresnel zones formed in a portion having a radius of 15 mm or more from the central axis of the Fresnel lens out of the plurality of Fresnel zones, and
• PAC1 represents an average pitch in the radial direction of Fresnel zones formed in a portion having a radius of 15 mm or less from the central axis of the Fresnel lens excluding a first Fresnel zone located on a most central axis side of the Fresnel lens out of the plurality of Fresnel zones,
• wherein
• the aspherical surface is expressed by using the following expression:
• [
• Expression
• ⁢
• ⁢
• 1
• ]
• Z
• d
• =
• y
• 2
• /
• R
• d
• 1
• +
• 1
• -
• (
• 1
• +
• k
• d
• )
• ⁢
• y
• 2
• /
• R
• d
• 2
• +
• ∑
• i
• =
• 2
• 10
• ⁢
• A
• 2
• ⁢
• i
• ×
• y
• 2
• ⁢
• i
• where
• Zd represents a sag amount of the aspherical surface at a distance y from the central axis of the Fresnel lens,
• Rd represents a radius of curvature of a reference spherical surface,
• kd represents a conic constant, and
• A2i represents a (2×i)-order aspherical coefficient, and
• the following expression is satisfied:
• kd≤−5.0.