system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how come in movies when the cameraman is filming the actor looking in the mirror, you can't see the camera even if the camera should logically be reflected in the mirror?

This question has been plaguing me for more years than it should have.
gpt
It's a little known fact, but there are a group of vampires in hollywood who are recruited a bit like stunt doubles but for cameramen when they need to film scenes about mirrors.

There are also several alternate techniques that people who don't believe in vampires usually employ.

First and by far the most common, the director takes advantage of the angle of reflection. Light reflects off a mirror in the same way a tennis ball bounces off a wall - that is, it only comes straight back at you if it hits perpendicular to the wall. For something to be seen in a mirror, light first needs to bounce off the object (in this case the cameraman), then bounce off the mirror straight back into the camera. When the camera is off to the side though, the light bounces in the opposite angle, so the camera doesn't see the camera in the mirror. This is why most mirror scenes are filmed at an angle, rather than directly behind the actor in question.

Secondly, you can not have a reflection at all, and construct the entire reflected scene in CGI. 

Thirdly, a technique done in Terminator when they wanted to do a panning shot around a mirror. They used a wall of glass instead of a mirror, put the actors in a duplicate of the set on the other side of the mirror then used dummies on the "real" side.