D3100 Video focusing problem

[Ray7545]

The camera initially focuses OK but when trying to zoom it goes all blurry and out of focus. Help please cant find anything in manual that would help.

 

[KmH]

In still shot mode the D3100 uses Phase-Detection auto focus. The Phase-Detection AF module is in the bottom of the camera, and a secondary mirror behind the main mirror doects the image down to the AF modulw.

In Live View/Movie mode the main and secondary mirrors have to be raised up out of the light path and the camera then uses Contrast-Detection auto focus. Contrast-Detection does not work as good for AF as Phase-Detection does.
You can select AF-Area modes for Liveview/Movie in the camera Shooting Menu - page 131 of your D3100 user's manual (the PDF file on the software disc)
Autofocus - Wikipedia, the free encyclopedia

 

[grafxman]

Yeh, the same happens with my Canon 7D. I've learned to stop down the lens to f13 or smaller which helps a little. I set the camera up so I can use the back focus button on the rear of the camera. That way I can zoom a little and focus a little at about the same time. Hope this helps.