Although I’ve never used a

Although I’ve never used a Canon C100, I can say that the size of the lens and the maximum f-stop has nothing to do with directly adding unwanted grain. The grain is a result of increasing the electronic sensitivity of the camera’s imaging device. Check the camera’s ‘gain’ selector or ‘ISO’ setting. When you boost these settings to a higher level, you are able to shoot in lower light situations but at the cost of increased grain. True, a faster lens means you won’t have to boost the gain as much, but the lens itself does not add grain. Perhaps the camera is on auto and is boosting the gain by itself…try switching to manual and turn the gain right down. Adding additional lighting can make a huge difference. My motto…”When in doubt, light it!”