fisheye with no distortion is no more oxymoron than rectilinear with no distortion. it is not possible to map 3d world as we see it (hemisphere) on a plane with no distortion. from this point of view, rectilinear is as bad as fisheye, just different. when speaking of lenses, "no distortion" usually means "no deviation from the idealized mapping", for rectilinear being x=t*tan(theta), and for fisheye x=r*theta or x=r*sin(theta).
best, mishka
