Not all lenses are good lenses, especially not the ones nature makes by accident. It also matters where the background source is relative the centre to the crap

Not all lenses are good lenses, especially not the ones nature makes by accident. It also matters where the background source is relative the centre to the crappy lens, see third picture here for how the same basic shape can make different images of the same source: http://inspirehep.net/record/850223/plots# http://inspirehep.net/record/850223/plots#