Total control over light, focus, frame rate and everything else is a filmmakers dream. When shooting with an iPad or iPhone that's nothing you can take for granted. FiLMiC is probably the best video camera app you can get.
I have borrowed the description of FiLMiC from its website:
"FiLMiC Pro turns your iOS camera into a broadcast worthy High Definition video camera. FiLMiC Pro variable speed zoom, audio gain control, variable frame rates up to 240fps on newest devices. With full manual control over focus, exposure, ISO, shutter speed, tint and color temperature you may never need another video camera again. Additionally, FiLMiC Pro has a ton of professional tools such as audio meters, stereo microphone support, informative thirds guide and aspect ratio overlays. And for the truly creative a variety of resolutions and aspect ratios including 2K, 3K and 4K on the newest iPhones; Cinemascope, Letterbox, Super 35 and Super 16 aspect ratios."
In addition to all the control FiLMiC gives you, I appreciate the unobtrusive interface that shows only the most essential information. You can, of course, tailor it after your needs.
The app has won over 5 000 dollar cameras in blind audience testing and tied against the 13 000 dollar Canon C300. It was used to shoot the Sundance Film Festival winner Tangerine on an iPhone 5S.
FiLMiC will cost you 15 USD.