What is a spy lens? how does it work?

Question

What is a spy lens? how does it work?

Answer 1

The old spy lenses were a tube that fit over the front of the lens and had a mirror in it, so that you could aim the camera one way, yet take a photo at 90 degrees to the apparent direction the camera was pointed.  It was a big deal in the sixties when guys (mostly) wanted to take shots of hot women at outdoor concerts without them catching them.. As I recall the didn't cost much more than a UV filter.