A learning experience with iOS's fingerprint recognition
I have both an iPhone and an iPad, both of which have fingerprint based unlocking, which I use. I interact with the iPhone sufficiently often that I generally unlock it multiple times a day, but for various reasons I use the iPad much less frequently and can even go for a couple of days before I dig it out and poke at it.
It's been winter around here for the past while, and Toronto's winter is dry. These days that dryness is hard on my fingers, especially the fingers of my right hand (I'm right handed, which may contribute to this); my fingertips get chapped and cracked and generally a bit beaten up despite some effort to take care of them by slathering moisturizer on and so on.
(The problem with using moisturizer, especially on your fingertips, is that I generally want to do something with my hands and don't want to get moisturizer all over what I'll be typing on or holding or whatever.)
Over the course of this winter, I gradually noticed that my iPad was getting harder and harder to unlock. I'd have to wiggle my right thumb around to get it to like it, and sometimes it just wouldn't and I'd wind up typing my unlock password. If I remembered to try my left thumb, often that would work, and my iPhone had no problems at all; I'd tap it and pretty much it'd always unlock. For most of the winter, when this happened I'd wipe the sensor clean on the iPad and mutter to myself and just live with it. It had to be a little glitch on the iPad, right? But every so often I'd stare at my roughed-up and increasingly hard to make out right thumb fingerprint and wonder.
When I couldn't unlock the iPad recently, I gave in to frustration and tried something just to see if it would help: I enrolled my right thumb's fingerprint again (as a new fingerprint). The difference was night and day. Suddenly the iPad was unlocking just like my iPhone, like it was supposed to and as I remembered it doing in the beginning; tap the sensor and it unlocked instantly without fuss or problems.
My best guess is the obvious guess; not only does the iOS fingerprint system have some margin for error, but it updates its recognition model over time. Because I unlocked my iPhone often enough, its recognition model could follow along as my right thumb's fingerprint got more and more roughed up over the course of the winter. However I didn't unlock my iPad often enough for these updates to kick in (or they couldn't or didn't move the model fast enough), so as the model and my fingerprint drifted further and further apart it got harder and harder to get it to match up with my cracked-skin fingerprint. Re-enrolling my thumb again added a new recognition model that worked on the current, beaten up state.
(This time around I've actually named that fingerprint, so I can easily remove it later. I may try the experiment of removing it in the summer when my right thumb's fingerprint is all recovered and has good skin condition again. In theory the original enrollment should be good enough at that point.)
Next winter I'm going to try to use my iPad more often or at least unlock it more regularly. Probably I'll aim to unlock it a couple of times every day, even if I'm not going to do anything more than tell it to check for various sorts of updates.
(Or I could start reading books on it. I did recently get pulled into reading a great SF novella on it, which was a pretty good experience, and I certainly have more books I could read there.)