Title text:
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.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3182/
Also known as the cabinet of curiosities, the door to the world that should not be seen, Medusa’s bane.