In iOS 10, you can adjust the brightness of the built-in flashlight if you have a 3D Touch capable iPhone. That rules out all iPads, iPhone SE and all iPhones prior to iPhone 6s.
Activate Control Panel by swiping upward from the bottom of the screen. Instead of tapping on the Flashlight icon, press on it with 3D touch. A menu will pop up with 3 options: Low Light, Medium Light and Bright Light. Tap to select your desired brightness.