[Steve Sharkey]

Generally speaking, you should leave the shutter speed at 50 when shooting video. You might also find that 400 is more like the D7100’s ‘native’ setting, going higher in low light and lower (or drop an ND filter on) when it’s brighter. Try those and see how you get on. Other than the focus not really following the subject, I’m not sure what distortion you’re referring to. An f-stop around f3.5/f4 should make it easier to keep a moving subject sharp – f1.4 really is very shallow. Are you depending on the camera to auto-focus?