Throw ratio and throw distance

Throw ratio = throw distance ÷ image width. A projector 20 m from a screen showing an image 10 m wide has a throw ratio of 2.0. Turn it round and the same formula gives the distance: image width × throw ratio.

Cinema projector lenses are sold by throw-ratio range, for example a zoom that covers a band of ratios. You pick the lens whose range contains your ratio, ideally not at either end of it. Manufacturers publish calculators for this; Barco's lens calculator takes the projector model, screen dimensions and distance and lists the lenses that fit.

Two formats from one position. On a constant-height screen the scope image is wider than the flat one, so the lens must cover both ratios from the same throw. Most digital installations switch with a motorised zoom and stored lens positions. Check both widths before buying the lens, using the aspect ratios you actually play.

Short throws cost picture quality. UNIC’s design handbook notes that a shorter projection throw means a shorter focal length, less depth of focus, possibly less sharpness, and more projection distortion. It sets a keystone limit of under 3%, with up to 5% acceptable, which constrains how high or off-centre the projector can sit.

Throw ratio also feeds brightness. A longer throw does not dim the image by itself, but the lens choice, port glass and screen size all affect what reaches the seats; the screen brightness calculator handles that side, and the screen size calculator sizes the image from the room.