What does lens offset refer to in projector specifications?
Except for very special short throw lens the projected image is not centered on the lens of the projector (zero lens offset). For the vast majority of commodity projectors the image is raised on the lens, typical lens offsets are between 90% and 110%. A 100% lens offset sees the base of the image pass through the center of the lens. Some projectors have a mechanical lens shift option, this allows the user to vary the degree of lens shift while retaining a rectangular image on a flat projection surface.Lens offset is relevant for spherical mirror projection into planetarium style domes because it impacts on the relative projector/mirror height in order for the light refected off the bottom rim of the mirror to pass over the top of the projector on its way to the spring line of the dome.