Finder optical system, image pickup apparatus, and image pickup system

A finder optical system comprising:

• a focusing glass including a collection surface configured to collect a light beam obtained via an image pickup optical system and a diffusion surface configured to diffuse the light beam collected by the collection surface;
• an eyepiece configured to receive apart of the light beam diffused by the diffusion surface;
• a photometry lens arranged near the eyepiece;
• and
• a photometry sensor on which an image of another part of the light beam diffused by the diffusion surface is formed via the photometry lens,
• wherein a plurality of micro lenses are arranged on the diffusion surface of the focusing glass,
• wherein at least one micro lens of the plurality of micro lenses has at least two radii of curvature on its cross section in a predetermined direction, and
• wherein the at least one micro lens satisfies R1<R2 where R1 is a radius of curvature in a first area of the micro lens where a distance between the first area and the photometry lens is smaller than a distance between a center of the micro lens and the photometry lens, and R2 is a radius of curvature in a second area of the micro lens where a distance between the second area and the photometry lens is greater than the distance between the center of the micro lens and the photometry lens.