I shooted some videos with the d3200 18-55 lens. To reduce the rolling shutter i preferred to shot at 1280x720p 60fps instead of 1920×1080 25/30 fps.
Less lines to scan for Expeed3 and better video.

To obtain the correct film back (sensor covered by video) i made different tests because the real value is hidden on the nikon site.
I shooted a static video at 18 mm looking at a checkerboard. I needed a method to obtain the width and height of film back.

I set the width to the maximum of the sensor 23.2 mm and I tried to calculate the focal lenght based on accidental perspective. So i had 18 mm confirming the correct width. The height, as a result, was 13.05 mm.

I was afraid if the sensor covered was really the center area (to reuse the distorsion calculated with stills) so i calculated also the distorsion zoom curve for each focal lenght.

Here the shots extracted from the video. All static so no rolling shutter.