So I'm actually a bit asahmed I have to ask this... but here goes. I understand about aperture and shutter speed. I understand, the faster the shutter, the lower the aperture and vice verse. I understand how to shoot on manuel and such. But what I don't understand is what someone means when they say to up or lower the exposure by a stop.. or 2/3 of a stop. Like, I understand that means "Change the Aperture." But how much exactly is a "stop"?