4.3 Lens shift

Lens shift moves the image without moving or tilting the projector. It is expressed as a percentage of image size, not as a distance:

Shift V is a percentage of image height. Positive is up.
Shift H is a percentage of image width. Positive is to the projector’s right.
Shift V = −50 puts the top edge of the image exactly on the lens axis.

Shift is almost always better than tilt. A shifted image stays rectangular; a tilted one becomes a trapezoid (§4.6).

Worked example — Demo2’s shifted machines

Machine	Epson EB-L695SE, WUXGA 1920 × 1200
Throw ratio	0.50
Throw distance	3.448 m
Image (3.448 ÷ 0.50, ÷ 1.6)	6.896 × 4.310 m
Shift V	−50 %
What −50 % buys	the image’s top edge lands exactly on the lens axis
Pixel density	278 px/m

These machines are also tilted, at 37.0° incidence, which is why their keystone widths do not come out equal — 6.922 m across the top against 7.772 m across the bottom (§4.5). The eight machines of the main array are the other case: no shift, no tilt, and top and bottom both 6.557 m.