4.2 Throw ratio and image size
The throw ratio is the one number that ties distance to image size:
Throw ratio = axial distance ÷ image width. So at a distance
d, the image is w = d / TR wide, and
h = w / aspect tall.
A zoom lens has a range of throw ratios; the inspector clamps what you type to the range of the lens actually mounted. A shorter ratio means a bigger image from the same spot.
Light does not leave from the middle of the chassis. Throwcast throws from the lens — the projector’s position plus the lens protrusion along its forward axis (0.06 m on the reference machines). Ignoring that makes footprints roughly 2.5 % too wide, which is enough to lose a blend. Folded optics — ultra-short-throw, right-angle and rear-throwing lenses — carry a full exit pose instead, so the beam can leave sideways or from a virtual point inside the body.