In reply to MG:
I like to think about how a pinhole camera works (directing points of light out in space to a specific corresponding position on the imaging surface) and then think of a lens as a craftily shaped piece of glass that implements the idea of a pinhole, but does it over a wider cross-section (aperture) so as to collect more light and make a brighter image.
Edit: oh yeah, but the point is that the crafty lens only works like a super light-collecting pinhole for light from things - and to an image plane - at a certain (focal) distance.
Post edited at 08:39