Radial distortion — where it appears
Straight lines that are not
Everybody says the edges of a wide-angle frame bow. Nothing is special about the edge. A radial map moves every point along its own radius, so the only line it leaves straight is one through the principal point, and the bend of every other is decided by how far it passes from that one place.
A lens destroys the invariant
The cross-ratio is the one thing a projection preserves, and everything checkable on this site is checked with it. A lens returns it one and a third per cent out where the pinhole is exact to fifteen digits — and the height error that follows tracks a quantity nobody would guess.
Named alongside it
The objects these essays reach for when they reach for this one.
Focal lengthPrincipal pointBarrel distortionBrown–ConradyCross ratioerror propagationHorizonPincushion distortionProjective invariantResidualSensitivitysingle-view metrology