Epson 1710c - Manual (Page 80)

80 Projector Installation
If you’re mounting the projector on the ceiling and you place it above
screen level, you’ll have to tilt the projector down to position the
image on the screen. This causes the image to lose its square shape.
You can correct the distortion using the
Keystone function in the
projector’s menu system (see page 49).
If you place the projector so that the base of the lens is level with the
bottom of the screen, the image should be square and you won’t have
to adjust it. If you place the lens lower than the bottom of the screen,
you’ll have to tilt the projector upward if you want to fill the whole
screen with the image.