Throw ratio is distance divided by image width: a 1.5:1 lens needs 4.5 meters to paint a 3-meter picture. Short-throw lenses go under 1:1, filling walls from a table’s length; long throws reach across ballrooms from booths.
The number decides gigs. A projector that cannot back up far enough produces a small image; one too far produces spill and dimness. Venue scouting for projection is largely measuring where a projector can physically live.
Throw interacts with brightness, since light spreads with area: doubling image width quarters the illumination. The planning trio is always throw, lumens, and surface, solved together.