Shooting movies with DSLRs (D3100)

[David8]

Hi guys,

So I know DSLR's aren't "video cameras", and I know that I'm not going to get the absolute best quality video out of a DSLR. That's not why I'm here. I want to confirm I think is true, but I dislike... I've had a Google, and I've read through my manual (although I admit I may have missed something). But to autofocus on a subject during shooting, do I need to release and half depress the shutter button again? Isn't there something about face tracking that should track movement and adjust focus accordingly? Is there a setting that I just haven't enabled, could it be that simple?

Any help would be much appreciated.

PS: The main use for this would be to put the camera on a tripod and start filming, and basically walk away...

Is it even possible?

David

 

[KmH]

The auto focus module in most DSLR cameras is in the bottom of the camera body. That AF module uses a phase-detection AF method.

To get light from the lens down to the AF module, there is a smaller secondary mirror behind the 50% transparent main mirror that directs light down to the AF module. the other 505 of the light goes up to the viewfinder.

To shoot video, both the main mirror and secondary mirror have to be moved up out of the light path so light can get to the image sensor. Consequently the AF module in the bottom of the camera cannot be used when shooting video. So in D-movie mode auto focusing has to be done in a different way, which is contrast detection.
The D3100 is able to do very fast contrast detection AF because of the EXPEED 2 image processor it has, but contrast-detection is still slower than phase-detection AF.

At any rate you have to choose from 1 of 3 AF Area modes - Nikon | Imaging Products | Digital SLR Camera Basics | AF-Area Mode (Live View Photography)
Then you have to select the AF mode - Nikon | Imaging Products | Digital SLR Camera Basics | Focus Mode (Live View Photography)

So to 'walk away' you need to select the Wide area mode and the full Time Servo mode - AF-F.

Nikon | Imaging Products | Digital SLR Camera Basics | Shutter and Aperture Construction
Understanding Camera Autofocus