It works by taking a picture, analysing the ambient light and then setting the correct flash tone and intensity. As you can imagine this takes time and there's a noticeable lag between pushing the button and taking the snap.
So there's a delay, but does it make a difference to the quality of the shot? As the comparison above shows, the difference is subtle but it is there. The 5S shot on the left has fewer shiny, harsh light reflections, while the hair colour is more natural and accurate. It's by no means a revolution in flash photography on a phone, not least as the range is still much the same, but it does diminish the worst imperfections caused by pure white LED flashes.
Video performance is also solid, with a few nifty additions. A slow-motion mode has been added and it's bundles of fun. The new camera and app allows shooting at 720p 120 FPS. Once you've finished shooting you can move some sliders around and make any section move in slow motion. It's simple but it works extremely well. Bouncing on a trampoline will never look more elegant.
All-in-all the myriad tweaks Apple has made to the camera of the iPhone 5S make it one of the best all-rounders out there, a fact helped no end by the much improved camera app that's easier to use and adds a few nice new features such as photo filters. It's hard to say it's the best phone camera around as the definition varies depending on your needs, but it's right up there.