Patent ID: 8425044

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, such that conditional formula (1A) below is satisfied, and by said focus adjustment, a distance between an exit pupil of rays incident on the screen upper ends of a screen surface and an exit pupil of rays incident on the screen lower ends of the screen is changed: 0.0090≦{(δ1−δ2)β2}/{(δ1+δ2)β1}<0.2 (1A) where δ1 represents, when an absolute value of a projection magnification is highest during focus adjustment, an absolute value of a difference between a distance from an exit pupil of rays incident on screen upper ends of the screen surface to the screen surface and a distance from an exit pupil of rays incident on screen lower ends of the screen surface to the screen surface, δ2 represents, when the absolute value of the projection magnification is lowest during focus adjustment, the absolute value of the difference between the distance from the exit pupil of rays incident on the screen upper ends of the screen surface to the screen surface and the distance from the exit pupil of rays incident on the screen lower ends of the screen surface to the screen surface, β1 represents a maximum value of the projection magnification when focus is adjusted (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 when focus is adjusted (except that (β2 represents, if the projection magnification is negative, a value of the projection magnification when the absolute value thereof is lowest).