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

Portrait Mode doesn't work well, or at all, in low light. If it's too dark for the feature to work, a message will appear within your camera app letting you know.

It also won't work if you're too close to the subject you're trying to capture, which your phone will alert you to as well. In order to take a Portrait Mode photo, you can't be any closer than 19 inches away from your subject.

It's also worth noting that Portrait Mode works best when there's a lot of contrast between the subject and the background. If you're shooting a white coffee cup on a white counter, for instance, the sensors may have trouble deciding what should be in focus and what should be blurred.

The image above demonstrates what a standard iPhone photo looks like (left) and what a Portrait Mode photo looks like (right). At a quick glance, the image on the right seems like it just has a totally blurry background, but this is where the depth map comes into play.

In order to make the photo look natural and as close to a real DSLR photo as possible, Apple's image processor goes through the layers one by one and blurs them in varying amounts, an effect known as "bokeh."

The layers closer to the subject will be slightly sharper than than the layers farthest away, and if you look closely at the above photo of my colleague Melia Robinson, you can tell: The stuff that's close to her in the photo — like the long grass and the slab of wood on the ground — is a lot easier to make out than the cliff in the distance, which is just a dark, blurry form.

When taking a Portrait Mode photo, the two lenses serve different purposes.

The telephoto lens is what actually captures the image. While it's doing that, the wide-angle lens is busy capturing data about how far away the subject is, which it then uses to create a nine-layer depth map.

Soon after Apple introduced Portrait Mode in 2016, the feature started popping up on other flagship phones like Samsung's Galaxy Note 8 (called Live Focus) and the Google Pixel (called Lens Blur).

In the case of the Pixel phones, which only have one lens, Google relies on software to achieve that Portrait Mode quality. Apple's iPhones require two lenses to make it happen — at least for now. So if you buy the new iPhone 8, for instance, it will not have the ability to take Portrait Mode photos.