Touch ID is Apple's fingerprint identity sensor, currently only available on the iPhone 5s. It allows you to unlock your device and make purchases from the iTunes and App Stores by touching and briefly holding your finger against the Home button. When it works, it's magical, and you want it everywhere - your iPad, your car, your house. When it doesn't, you want to throw your iPhone across the room. What can make Touch ID failures even more frustrating is figuring out why they're failing. From the outside it seems like the stereotypical black box. A fingerprint goes in and then it either works or doesn't, and if it doesn't, there's no way for us to see what went wrong, why, or how to fix it. Maddening. There are, however, some things you can do to make it work better and more often...

When the capacitive ring around the Touch ID sensor detects a finger, a triggers a high-resolution image capture. That image is converted into a mathematical representation, which is then sent through the hardware channel to a secure enclave on the Apple A7 chipset. If the representation matches what's stored in the enclave, a "yes" token is released and the unlock is authenticated or the purchase authorized. If not, a "no" token is released, and all you get is a digital head shake. Every time Touch ID scans a finger and recognizes it, it adds additional detail to the representation, theoretically to make it register even faster and better in the future.

So what can go wrong, and how can we prevent it from going wrong?

  1. During the registration process, make sure you move your finger around enough that the entire surface gets scanned during the first stage, and then every edge gets scanned during the second phase. When you're setting up Touch ID, you're right in front of your phone. When you're using it in the real world, you'll be coming at it from every angle.
  2. Any sweat or liquid on your finger at all can interfere with the scan. Wipe both your finger and the Home button off and dry then completely before using Touch ID.
  3. The sequential improvement process can sometimes go off-track (i.e. instead of getting better, an error can occur and it can get worse). When that starts to happen, delete the fingerprint and re-register. That'll return it to its base state and prevent chaos from swallowing the process whole. (Guy English had some smart ideas about how Apple could improve this part, hopefully he'll write them up...) (Hi Guy!)

There's a theory on the 'net that registering the same finger more than once, including variants like doing it again upside down, can improve Touch ID speed and reliability. I don't have enough information to determine whether, long term, that's true or not. Since Touch ID is always updating registered prints, I don't know if conflicts or collisions can occur.

Currently, I have 3 fingers registered, no duplicates. I re-registered them all once about a month ago, and Touch ID works for me immediately about 90% of the time, after 2-3 tries 8% of the time, and fails completely (requiring me to enter a Passcode) about 2% of the time (not counting reboots when Passcode is mandatory). However, I'm aware of people who have much, much lower success rates, and how rage inducing it is when you're denied the future today.

Let me know how Touch ID is working for you in the poll above, give me the details in the comments below, and if you have any Touch ID tips to share, please do!

Update: Serenity Caldwell of Macworld has posted some excellent trouble-shooting tips for Touch ID. Check them out!