#3182: Telescope Types

XKCD comic, described below.
Transcript
Ambox warning green construction.svg This is one of 34 incomplete transcripts:
Don't remove this notice too soon. If you can fix this issue, edit the page!

(Sourced from explainxkcd.com)

I'm trying to buy a gravitational lens for my camera, but I can't tell if the manufacturers are listing comoving focal length or proper focal length.

ExplainOriginal