"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?
Stopping the lens down in Aperture priority movie mode does increase the dof just as it does for a still photo. However you need to set the aperture prior to turning on Liveview. The aperture won't change to a new setting until you turn off Liveview, reset the aperture and then turn Liveview on again. You can't change the aperture while recording.
The reason the LCD stays bright is the gain to the LCD is increased in order to keep a constant brightness over a wide range of ambient light.
I normally don't shoot videos but of the 3 or 4 I've taken this is the way the camera reacted to different f stops. I only use A mode so I don't know how it would react in P, M or M mode.
I know there are a few on this forum that take videos all the time. They might be able to expand on my experience.
The lens won't stop down to a new aperture while in Live View, so, no, DOF and exposure won't change. As Len has already mentioned, you need to set the aperture while out of Live View, i.e. it won't respond to a change while in Live View.
I understand that you can't change it in Live View, but if you set it to, let's say, f/18 in aperture view, will it shoot video at f/18 when you go back to live view? Or will it just go back to wide open?
>I understand that you can't change it in Live View, but if >you set it to, let's say, f/18 in aperture view, will it shoot >video at f/18 when you go back to live view? Or will it just >go back to wide open?
It will shoot the video at f18. This is very easy to check. First use a lens where you can see the aperture blades when it's stopped down. Then set the aperture to f18 or f22. Turn on Liveview. Then turn on movie mode and start recording. If you now look into the lens you'll see the aperture is stopped down while the movie is recording.
Here is a photo of my D5100. The lens is an AFS 35mm f1.8G. The camera is in Aperture priority mode and the lens is set to f22. The camera has Liveview on and is recording a movie in Movie mode. As you can see the lens is stopped down while recording.