"Back in the day", I would select the focus, apperture and shutter speed on my SLR. When I fired the shot, the mirror would go up, the apperture would stop down to the selected setting and the shutter would open for its selected setting and the process would reverse back to mirror down and lens wide open. If one wanted to get an idea of the depth of field, there was a "Preview" button that stopped the apperture down to its preset value. When this was the done, the view in the viewfinder darkened.
For photography with the d5100, I assume the same process holds (except everyting is automatic unless camera is told otherwise). However, when I was playing with the video function of the 5100, I set ISO at 800 and opened the lens up all the way. I could watch in LiveView at a certain brightness. I then set the lens at f/18. The brigtness of the LiveView and the resultant video didn't seem to change. This was indoors at night with just a room light. I would have expected the picture to go almost black when the lens was stopped down. (I was in Apperture Priority when I adjusted the apperture setting). Does the apperture setting not apply in video mode?