How do I password-protect access to specific apps in iOS?
The only way to passcode protect individual applications, including built in system ones is to jailbreak the phone and install a jailbreak tweak like Locktopus or Lockdown Pro. Both of these tweaks let you set a text passcode as well as a numerical one. The reason an AppStore application cannot provide this functionality is because AppStore apps are Sandboxed. This means they are limited in how they can interact with other applications, and the iPhone itself.
Alternatively, replace the application you are currently using with an AppStore alternative that has a passcode lock built into the application. This wont protect the data in the original app, but if you want to secure data behind a passcode you could move the data into the passcode protected application and delete it from the original application. This obviously wont work for contacts, but could work for photos and notes.
See the iOS application &quot;MyPics&quot; for an example of a photo app where you can set a passcode to protect access. Another example is the My Secret Folder application which lets you import data into it and set a passcode to prevent people from accessing that data within the app.