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(Pocket-lint) - Apple announced new details about how Face ID works... via an email.

The iPhone X's exclusive facial-recognition feature will work with most sunglasses, according to Apple's software engineering chief, Craig Federighi, who has replied to an email from developer Keith Krimbel (@keithkrimbel). "Most sunglasses let through enough IR light that Face ID can see your eyes even when the glasses appear to be opaque," explained Federighi. "It's really amazing!"

When Apple announced the feature on 12 September, the company said it would recognise you even if your appearance abruptly changes due to things like hats, scarves, beards, glasses, makeup, and other items. Sunglasses, however, were not specifically mentioned by the company, but now, thanks to Federighi's email response, we know that those won't be an issue either. Phew.

Krimbel also asked Federighi what would stop a thief from taking someone's iPhone X, then pointing it at their face, and running off with the unlocked device. Federighi said Apple has two solutions in place: "If you don't stare at the phone, it won't unlock," he said. "Also, if you grip the buttons on both sides of the phone when [you] hand it over, it will temporarily disable Face ID."

So, squeezing the buttons on both sides of iPhone X disables Face ID. Cool. And, finally, Federighi commented on how Face ID seemed to fail during a demo during Apple's event. Apple has already explained that the software failed because someone else had picked up the phone ahead of the demo. Federighi told Krimbel that he was shocked when Face ID didn't work for him:

"The bio-lockout that I experienced on stage would require several interacts by other people with your phone (where they woke up the phone). For those of us who have been living on the iPhone X over the last months this has never been a real problem (hence my shock when it happened to me on stage! :-)"

Face ID is meant to replace Touch ID as a biometric authentication system. Apple has described it is the future of how we will unlock our smartphones. You can read more about how Face ID works from here.

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Writing by Elyse Betters.