Here's how you can set your iPhone 5 or 4S camera to automatically use flash when needed while taking photos.
1. Open the camera app.
2. Tap the flash button in the upper left corner and select Auto.
3. When light levels are low enough, your iPhone 5 or 4S camera will automatically use the flash to light up your photos.
4. To turn it off, just tap the button and set it to Off.
5. To have your camera use the flash each time, set it to On.