Where the Projector Has to Go: Throw Distance, Lens Shift and Keystone

A projector's throw ratio is the distance from the lens to the screen divided by the image width. A 1.5:1 ratio means a 100-inch 16:9 screen — 87 inches wide — needs about 131 inches, near enough 11 feet, of clearance. A zoom lens turns that into a range: a 1.6x zoom over a 1.35 to 2.16 ratio puts the same screen anywhere between about 10 and 16 feet out.

Measure the room before shortlisting projectors. If the only mounting point is 9 feet from the wall and you want a 120-inch screen, you need a projector whose throw ratio can go down to about 0.9, which rules out most of the home-theater tier and points you at short throw. Every projector page here computes the range both ways from the maker's own throw ratio.

Projectors have a fixed vertical offset: the image does not come out centred on the lens, it comes out above or below it depending on orientation. Optical lens shift moves the lens assembly relative to the imaging device, sliding the image up, down or sideways with no distortion and no loss of resolution. It is the difference between a projector you can put where the room allows and one that has to sit on the screen's centre line.

Ranges vary enormously. Some projectors in this catalog have none at all. Some offer ±10% vertically, which is enough to clear a coffee table. The 3LCD and LCoS home-theater models go to ±96% vertical and ±47% horizontal, which will hit a screen from almost any shelf, ceiling or back-wall position. If the mounting position is constrained, this is the spec to shop on.

Digital keystone correction squeezes the image in software to compensate for a projector that is not square to the screen. It works, it is on almost every projector sold, and it costs you pixels: the correction throws away part of the imaging device's resolution and resamples the rest, which softens the picture. On a 4K projector you are discarding some of what you paid for.

Automatic keystone on lifestyle and portable projectors is a genuine convenience and the right call for something you move around nightly. On a permanently installed projector it is a sign the install is wrong. Square the projector to the screen, use optical shift for the offset, and leave keystone at zero.

Ceiling height and the mount drop: a projector hanging 18 inches below an 8-foot ceiling is 6.5 feet up, and a 120-inch screen's top edge is near 7 feet. Those numbers have to be reconciled before anything is drilled. Check what the lens's vertical offset actually is in ceiling orientation; many projectors throw well below their own axis when inverted.

Then power and signal. A ceiling-mounted projector needs an outlet at the mount and an HDMI run that will pass whatever bandwidth you need — a 4K/120 signal over a long run is a real cable problem, not a theoretical one. And leave service clearance: a lamp-based projector will need its lamp changed, and the filter on almost any projector needs cleaning.