Non-perfect lens: aberrations
Simple lenses are not ideal:
- usually one object point creates not an image point but a spot
(as the diameter of the lenses are bigger, this error is more and more obvious)
- different color light rays are producing different images
Correcting for aberrations:
- making lens by combining many lens (diopters simply add)
- making lens with special covers
- making lens with special and not spherical surfaces
Using sometimes curved mirrors instead of lenses (no color aberrations)
Aberration free lenses are very expensive ...