Patent ID: 8113667

Claim:
A projection optical system that receives light from a display device surface and enlarges and projects a display image thereon obliquely onto a screen surface and that varies a projection distance to a screen to display images of different projection magnifications, the projection optical system comprising: one or more reflective surfaces having an optical power between the display device surface and the screen surface, wherein focus is adjusted by moving at least one optical device having an optical power, and conditional formula (1)′ below is satisfied: 0.02<{(tan θ f 1−tan θ f 2)−(tan θ n 1−tan θ n 2)}·(β2/β1)<0.20 (1)′ where, when a screen of the display device surface is rectangular in shape, a direction of a normal to the screen of the screen surface is referred to as a “z-direction”, a direction of a long side of the screen of the screen surface is referred to as an “x-direction”, an x-z plane component of an angle of incidence with respect to the screen surface is referred to as an “incident angle θ (°)” and among rays that pass through a center of an aperture and that are incident on ends of upper and lower sides of the screen of the screen surface, the incident angle θ of rays whose incident angle θ is larger is referred to as “θf” and the incident angle θ of rays whose incident angle θ is smaller is referred to as “θn”, θf1 represents, when an absolute value of a projection magnification is highest during focus adjustment, the incident angle θf, θf2 represents, when the absolute value of the projection magnification is lowest during focus adjustment, the incident angle θf, θn1 represents, when the absolute value of the projection magnification is highest during focus adjustment, the incident angle θn, θn2 represents, when the absolute value of the projection magnification is lowest during focus adjustment, the incident angle θn, β1 represents a maximum value of the projection magnification during focus adjustment (except that β1 represents, if the projection magnification is negative, a value of the projection magnification when the absolute value thereof is highest) and β2 represents a minimum value of the projection magnification during focus adjustment (except that β2 represents, if the projection magnification is negative, a value of the projection magnification when the absolute value thereof is lowest).