Barrel 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.
Wide angle is not distortion
A wide lens stretches shapes at the edge of the frame by exactly 1/cos θ — 3% at 28° across, 41% at 90°. Every bit of that is what a correct rectilinear projection must do, and every bit of it disappears if the picture is viewed from the point it was made for. Nobody views it from there.
Named alongside it
The objects these essays reach for when they reach for this one.
AnamorphosisBrown–ConradyEdge stretchfield of viewFocal lengthPicture planePincushion distortionPrincipal pointRadial distortionRectilinear projectionViewing distanceVolume anamorphosis