My favorite site for explaining lens aberrations (unfortunately, it's currently under construction in a new URL) is here: https://web.archive.org/web/200902042

My favorite site for explaining lens aberrations (unfortunately, it's currently under construction in a new URL) is here: https://web.archive.org/web/20090204211139/http://toothwalker.org/optics.html https://web.archive.org/web/20090204211139/http://toothwalke... It's a fantastic resource.