Here's why Apple's 'Portrait Mode' feature only works on some iPhones and not others

When Apple introduced the iPhone 7 Plus last year, it contained a new camera feature that quickly became one of the most talked about - and copied - in Apple's lineup: Portrait mode.

Portrait mode uses the phone's dual cameras and Apple's software to mimic the quality you would get from a DSLR camera, which keeps the subject of the photo in focus and slightly blurs out the background.

The feature originally launched in beta as an exclusive for the iPhone 7 Plus. But now, a year later, Portrait Mode is live on the new iPhone 8 Plus (and will be available on the new iPhone X launching this fall).

Here's how Portrait Mode works, how you can use it, and why it's only available on some iPhones and not others: