Ah... wait... the flipped / compressed image is actually exactly what I was originally talking about / expecting to get...

...well, I'd forgotten the flipped part...

http://www.warren-wilson.edu/~physics/PhysPhotOfWeek/2007PPOW/20071130LensPPOW/index.html

...and this is exactly what I am now getting.

The magnifying glass prop in my scene is a bit of a distance from the camera.

It all makes much more sense now :)