Patent ID: 8218241

Claim:
A viewing optical system positioned between a viewing plane as a virtual surface and an eye point, characterized by comprising, in order from said viewing plane side, a first lens group including a positive lens, a second lens group including a cemented lens in which at least one negative lens and at least one positive lens are cemented together, and a third lens group including a positive lens, wherein: of lenses forming said first, second and third lens groups, at least one lens surface is composed of an aspheric surface, and said aspheric surface is configured such that a first normal at a first point on said surface intersects a second normal at a second point, and satisfies the following condition (1): 0 <ax/Ra< 1 (1) where ax is a distance on an optical axis from an apex of said aspheric surface to a third point, wherein, assuming that Y is a position where a farthest off-axis chief ray passes on said viewing plane, said first point is defined by a point at which an off-axis chief ray at a position 0.9 times (0.9Y) said position Y intersects said aspheric surface, said second point is defined by a point at which an off-axis chief ray at a position 0.7 times (0.7Y) said position Y intersects said aspheric surface, and N is defined by a point of intersection of said first normal and said second normal, said third point is defined by a point at which a straight line passing through said point of intersection N and vertical to said optical axis intersects said optical axis, and Ra is a radius of curvature of the apex of said aspheric surface.