Do you want to take full control over your iPhone camera?
You can: Apple's iOS provides access to all the functions that an enthusiast photographer might want, including ISO, shutter speeds, white balance, manual focus and so on.
Unfortunately, all these are just not available in the stock camera app, where the only thing you can manually control is the exposure. A new third-party app, Raw+, allows you to control all aforementioned camera controls fully manually, as well as shoot in Raw format.
Raw+ is a free-to-try app, but if you want to use the app after the initial 100-shot limit, you'd need to shell out $5 via an in-app purchase.