The screen finally feels how hard you press
3D Touch on the iPhone 6s, one year after I guessed pressure-sensitive phones were a couple of years away. How it measures force, and what it's for.
Last September, after the Apple Watch announcement, I wrote that pressure-sensitive screens would reach phones “within a couple of years.” It took one. The iPhone 6s has 3D Touch, and it’s paired with a Taptic Engine, so the phone taps back when you press.
As far as I can tell from Apple’s description and the early teardowns, it works by measuring how much the glass bends. There are capacitive sensors built into the backlight layer under the display. When you press, the cover glass deflects by a tiny amount, the distance between the glass and the sensors changes, and the capacitance changes with it. Combined with the touch sensor, which already knows where your finger is, the phone gets a location and a force for every touch.
The mechanical engineer in me finds this very satisfying. It’s a strain gauge problem. Glass is stiff, so the deflection is tiny, and it varies with where on the screen you press, since the middle of a panel bends more than the edges under the same force. Temperature changes the gap too. Getting a reliable, consistent force reading across the whole screen, on every phone coming off the line, is a calibration challenge I would not want to own. Apple seems to have done it.
The interesting question is what software does with it. Apple’s first uses are “peek and pop” (press lightly to preview a link or email, press harder to open it) and shortcut menus on app icons. Those are fine. They’re also basically right-click, which is a little underwhelming for a new input dimension.
The bigger opportunity, I think, is continuous control. A touch today is binary: down or up, at a location. Force is a value you can vary smoothly, so you could do things like drawing with line weight that follows pressure, scrubbing through video faster the harder you press, or controlling a game character’s speed. Musical instruments are the obvious case. A piano key’s force is most of the expression.
There’s also discoverability. Nothing on screen tells you that pressing harder will do something, and I’ve already seen friends not find the feature for days. Hidden gestures tend to stay hidden. The Taptic feedback helps once you know it’s there, since you feel a distinct click at the threshold. Getting people to try pressing in the first place is a design problem Apple hasn’t solved yet.
Our studio has already had two clients ask whether they should support it. My answer for now: yes for shortcuts, since it’s cheap. For anything more ambitious, wait and see whether users learn to press.