I would probably invest in a lighting kit if you dont have one already, your video seems fine to me but it looks like its shot in front of a window relying on daylight, there's a shadow across your face and body too with only your hands being illuminated fully. It seems as good as you can get more or less with the lens you're using in those lighting conditions. You could try boosting the ISO but its not a good thing to do if you're already shooting at 400. I would try not to go above 300 when possible. I have seen a few people mention that 200 is better than 100 for video although I cannot remember why exactly.