whats that thing with the white ball?

Question

during photoshoots i see this person with this thing with a white ball on it. what is it? how does it work?

Answer 1

That handheld device with the white ball is a light meter.
You use it to measure how much light the subject reflects, and calculate what shutter time/ aperture setting is best.
Cameras all have a built-in light meter these days, but those external devices provide a bit more accuracy.