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I haven't heard of many complaints about Touch ID on the iPhone 5s. However, if you're like me, you may have had some frustrations getting it to always work the first time. It's just plain annoying to have Touch ID fail to recognize your finger the first time, and if you fail three times, you'll then have to enter your regular pass code — something you could have entered in the first place with less hassle.
This is less than optimum when showing off Touch ID to friends. One doesn't want to get Doonesburried.
In my case, it seemed that I was only getting about a 70 percent success rate with my right index finger. (Less, of course, during my demos. Murphy's Law.) I tried to make sure my finger was clean and dry every time. That didn't help, and I wanted more.
After some initial trials, I followed the consensus advice to define a second finger, say, the thumb. If my index finger failed, I'd try the thumb. Often, after my index finger failed, my thumb would too. (I followed the advice about how to move the finger around during training — to no avail.)
I decided that the best thing to do was to bide my time and wait to see what developed in the Apple tech community.
A Eureka Moment
Yesterday, I tried defining my right index finger twice. That is, I deleted all previous fingerprints, started from scratch, and then defined my right index finger. I labelled it R.index1. Then I did it all over again and labelled the second one R.index2. Since then, I have been able to authenticate on the 5s with my right index finger the first time, every time with zero failures. None.
Discussion
My theory is that if the first definition fails, the 5s tries the second definition. You may have defined the second one for a different finger, but the 5s doesn't know which finger you'll try first. So perhaps it just tries all of them for each press.
By using the same finger with two definitions, if the first one fails, the second one might succeed by some quirk of the recognition algorithm. It's like rolling dice. If the chance of rolling a 6 is 1/6, then the chances of rolling two 6's in a row is 1/36. By analogy to the 5s, the chance of failure is greatly reduced with two full definitions (two rolls) of one finger.
Again, this is just my theory. What I do know is that for two days now, I have had zero failures when authenticating on my iPhone 5s. If I ever do get a failure, I'll definitely try adding a third definition for the same finger. That should reduce the failure rate even further.
If you've been frustrated with a less that perfect success rate, and this trick works for you, let us know in the comments below.
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Touch ID tech diagram via Apple.