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/
The good news, is after you lose everything at least once, life broadly becomes a lot less scary. Best wishes and I hope you land on your feet whatever the results.