Image Circle
The circular area of usable image a lens projects, which must be at least as large as the sensor's diagonal.
Formula / rule
Diameter ≥ sensor diagonal
Measured in
mm
In depth
Format compatibility is really a question of image circle: a full-frame lens projects at least 43 mm and covers a smaller APS-C sensor easily, while an APS-C lens projects around 28 mm and will vignette badly on full frame. Tilt-shift and view-camera lenses deliberately project far more than they need, and that surplus is exactly what the movements consume.
Examples in practice
A 24 mm shift lens projects a 60 mm image circle so it can be shifted 12 mm off-axis without darkening the corners.