While iPhone 5s's TouchID has been lauded for bringing biometric security to mainstream, it's not without flaws. A long time ago, there was a video showing how you can trick the iPhone 5s to accept all five fingers as inputs for just ONE fingerprint. Effectively, you could have all ten fingers working with just two fingerprints listed.
Now, after several months of iPhone 5s's release, users are coming up with issues relating to TouchID's performance. There's not one specific reason why this happens as the contexts are varied but let's try to pin down some dos and don'ts that will help you fix the TouchID if it isn't working fine.
Calibrate Accurately on iPhone 5s
One of the first mistakes happens when you calibrate the fingerprint. Even though the iPhone 5s tells you your finger has been calibrated successfully, the fact remains that there's a slight probability and guesswork involved.
When you calibrate, hold the iPhone 5s as you'd normally hold. Don't hold it “perfectly” because that's not how you hold it during normal use. Then, calibrate the finger.
When you calibrate, make sure all parts of your fingerprint are scanned – the most important thing is to scan the tips and edges of your fingerprint too.
No Damp Screens or Fingers
Dampness, moisture, sweat, cold – all of this plays a role in preventing the TouchID from working correctly. This happens both ways: if your finger is damp or if the home button has some moisture over it.
The solution is, obviously, to use clean fingers. Wipe your finger before you use it to authenticate/unlock the iPhone. Similarly, if you are using the iPhone 5s out in the cold, wipe the home button clean before you place your finger there.
Touch the Ring
The ring over the home button is the guy who figures out you are about to unlock your iPhone and the device should scan and check the fingerprint. So, if you happen to – very carefully – avoid touching the ring, the iPhone won't unlock.
Whatever you do, make sure you place the whole of your fingerprint over the home button, covering the rings too because that's when the sensors are triggered.
One Finger, One Fingerprint
Obviously, this is not supposed to be a valid point but seeing that you can use multiple fingers under one fingerprint, I think this should be taken a tad too seriously. Make sure you only calibrate one finger for one fingerprint listing.
Keep the Phone Clean
Keeping the phone clean goes beyond wiping the home button when you use it. The problem with iPhones is that dust can get under the home button and in case of an iPhone 5s, that's quite bad because the fingerprint sensors are down there.
For the iPhone 5s, we can't recommend any method to keep the home button clean and safe but a regular wiping with clean cloth. Going military-grade with cases isn't really a neat solution for most. Also, keep your iPhone in places where there's minimal dirt, dust or moisture.
Reset the Fingerprints on iPhone 5s
Finally, if the issue seems to be getting worse, it's time to delete the fingerprints and add them again.
TouchID can also be used for App Store purchases. If you want to turn off TouchID for App Store purchases and just use a passcode/password authentication instead, here's how to do that. Read some on TouchID's security and why it's one of the best mainstream applications of biometric sensors in mobile devices.
Here's a complete guide on adding and deleting fingerprints for TouchID on iPhone 5s.