My nikon d3100 won't focus whilst using live view? Only when i'm using the viewfinder?

I have recently got my d3100 and love it, but it seens that when shooting videonor just using live view it will not focus? I have it on auto and it obviously works when I do it manually, the only time it focuses is when i point it at a bright light source like my bedroom light, what's wrong with it?

You are right… Sort of. The D3100 can auto-focus in the video mode, but not nearly as fast as when using the camera as a still camera.

Had you done your research, you would have found that the video feature is just that, a feature on a still camera.

This is why it can get very expensive really fast when you start adding accessories to dSLR's to make them a viable video camera, up to $2, 500.

http://www.luminous-landscape.com/reviews/camcorders/cam-cam.shtml

What you are calling "bright light source, like my bedroom light" is NOT bright light. A bright light, especially when shooting indoors, would be a 500 or 750 watt incandescent lamp, but aiming your camera at the light source certainly will activate the auto focus in the video mode Even in the still mode, there's an auto-focus assist lamp that helps the lens focus.

It seems that you still need to learn the fundamentals of photography. You have unrealistic expectations of the tools you own.

I have owned my D3100 since 2011 and I converted it for infrared, so I must use LiveView for focusing. So if I focus using the viewfinder (assuming the IR filter is direct on sensor) then the photos are slightly out of focus (using autofocus) because there is a slight focal length difference between visible and IR. If I use an external (lens mounted) IR filter, I cannot use the viewfinder and MUST use the LiveView autofocus, and for some reason the autofocus is no longer working. I never changed any settings, but I have reset everything to factory settings for the purpose of troubleshooting, and I even removed the IR filter from the sensor and switched to Full-Spectrum and sometimes use an external IR filter. Even without any filters, no matter what lens I use, LiveView autofocus just does not work. It stopped working prior to removing the IR filter, so I'm confident it wasn't an error on my part, at least as far as my opening/closing the body and reattaching all the wires and ribbon connectors. Also, I just want to point out that I use the back button focus, the AE-L/AF-L button for autofocus, for the sake of ergonomics and steadiness. Using this method worked fine previously, but I've reset that with all the other settings. I also use a battery grip with shutter button, and neither shutter buttons being pressed partially will engage the autofocus. In my opinion, once the autofocus stops working, the camera needs to be put down and it's time to buy a new body. To me it seems the problem is within the electronics and can only be fixed by a professional, which is going to cost far more than the camera was worth at the time of purchase. For anyone that is really experienced with small electronics that wants to experiment with the components inside the camera, good luck and maybe that's a fun project for you, but to anyone else that doesn't have that skill, just shop around for another body that's compatible with your current lenses and equipment.