How to disable auto-gain during video recording?

Melillo

Hi,

I did my first experiment with D3100 video recording yersterday. The camera was mounted on a tripod and I filmed a family reunion in the living room. I noticed when people moved in the room, the brightness of the walls changed, as if there was some sort of automatic gain control on the camera. I used both M and A mode. Is there a way to disable that behavior?

Thanks in advance and have a nice day!

Marc