I'm just getting used to the manual controls on my secondhand Panasonic nvgs400 so forgive me if this is a silly question.

Last night I was trying to film someone in a chair, in a dark setting with a black background; There was only one small reading lamp, from the side, and this just lit up parts of the person and nothing around him; The automatic setting put the iris at open +15db; Using manual I took this down to open+0db; It was the effect I wanted but when the subject lit a cigarette lighter or even showed reflected light off his glasses the camera went out of focus until the flame or glint was removed.

So my question is, can anyone explain why?

Thank you in advance.