"Our technology analyzes the pixels in photos you're already tagged in and generates a string of numbers we call a template", said Joaquin Quiñonero Candela, Facebook's director of Applied Machine Learning, in the blog post.
The new feature is based on the same technology Facebook has used to suggest friends you may want to tag in photos or videos.
If you're in someone's profile photo which is always public, you'll always be notified.
Candela went on to say that Facebook will respect the privacy setting people select when posting a photo, meaning "you won't receive a notification if you're not in the audience". The idea is to give you more control over your identity online by informing you when your face appears in a photo, even those you don't know about.
This new setting will be able to be turned off with a simple on/off control. Although it does not appear to have been implemented yet, Facebook will soon allow users to turn off facial recognition on their accounts with a single switch.
Facebook will let you know when someone posts a photo of you - even if you aren't tagged in it. The goal of the scanning, according to Facebook, is to alert you if someone has publicly uploaded a photo of you that you don't know about, especially if they are trying to impersonate you. Finally, the company is also going to use facial recognition to describe pictures to the visually impaired; someone looking at a photo using a screen-reader can hear which of their friends are in the picture by name.
In conjunction with the new facial recognition tools, Facebook put out another post in its "Hard Questions" series.
"The words "face recognition" can make some people feel uneasy, conjuring dystopian scenes from science fiction", wrote Rob Sherman, Facebook's deputy chief privacy officer.
Facebook is also planning to roll out a "simple on/off switch" to replace the settings for individual features that use face-recognition technology. However, he argued against strict regulation of facial recognition technology, comparing current fears to similar concerns in the late 1800s over cameras. We're doing this to prevent people from impersonating others on Facebook.