I am trying to shoot a video with perfectly white background with a similar setup as described here: http://www.gowallaby.com/how-to-get-a-white-background-in-video/
However, Im using Nikon D3100 instead of iPhone to shoot the video. I can't find a way to lock the focus and light settings as instructed in the article
Watch for a yellow box to appear and bounce twice, and then indicate that the focus and lighting are locked. You can now turn on your background lights and you’ll see that the background becomes bright white.
I assume similar result should be possible with Nikon D3100 as with iPhone. Any suggestions?