Camera
The camera is one of the biggest areas to receive an upgrade in the Xiaomi Mi 6. We're seeing a dual rear sensor implementation similar to the iPhone 7 Plus here, with a normal wide angle camera paired with a 2x zoom camera. I tend to think this is one of the better dual camera solutions, as the zoom does come in handy if you want extra close-up detail.
Wide-angle camera
The actual hardware we're seeing is two 12-megapixel sensors on the rear, however the sensors aren't identical. The wide-angle camera gets a Sony IMX386 1/2.9" sensor with 1.25µm pixels, paired with a 27mm f/1.8 lens with optical image stabilization. The zoom camera has 1.0µm pixels, which equates to a 1/3.6" sensor, with a 52mm f/2.6 lens, meaning the actual zoom factor from wide to secondary camera is 1.93x.
2x zoom camera
There are some interesting things to note about this camera setup. Firstly, the zoom camera does not have OIS, which is arguably more useful on this camera as it is more prone to motion blur from a shaky hand. It seems there is simply no room (or budget) to include both a 52mm lens and OIS in the same module. It's also worth noting the zoom camera has less light gathering ability, as its lens is a full stop narrower and the sensor's pixels are smaller.
The front-facing camera is an 8-megapixel native 16:9 sensor paired with an f/2.2 lens. Video capture goes up to 4K at 30 frames per second on the rear camera, while the front camera is capable of 1080p video capture (despite having enough resolution for 4K).
The quality of the rear cameras on the Mi 6 are... okay. Up against the best cameras at the top of the tree - namely the Galaxy S8, Pixel XL, LG G6 and iPhone 7 Plus - the Mi 6 doesn't compete strongly as these cameras focus heavily on image post processing. The Mi 6 tends to use more stock-quality processing, which leads to weaker results in general.
Outdoors, the Mi 6 is a mix of great photos and surprisingly poor photos. When the sun is shining, either camera produces great results, with saturated colors and decent contrast. However, if it's even slightly cloudy, the Mi 6 tends to make images way too underexposed, leading to dark and unimpressive photos. Other flagships perform better across the board when outdoors, which is usually the strong point of post smartphone cameras.
Indoors, the Mi 6 again is a reasonable but not fantastic performer. The main 12-megapixel wide-angle camera exposes well and produces photos with respectable levels of dynamic range and color, however fine detail tends to suffer indoors due to post-processing artefacts. The Mi 6 is better than average in these conditions, and should suffice for most users.
Considering the Mi 6 boasts 1.25µm sensor pixels, an f/1.8 lens on its wide camera, and optical image stabilization I was expecting better results in low light. Images are bright and vibrant for the most part, however detail isn't great and it takes a steady hand to get a blur-free result, even with OIS. Perhaps the OIS module Xiaomi uses here isn't very effective.
HDR shot
Let's talk about some of the oddities of the Mi 6 camera. To start with, the camera application doesn't support automatic HDR, instead opting for a toggle in the main interface. My recommendation is to always leave the HDR mode on, as it tends to produce far better dynamic range and color quality, along with a slightly warmer tone. Default dynamic range from the sensor isn't fantastic, but the HDR mode does a great job of improving results across the board.
I wish Xiaomi used auto HDR because I think average consumers would be much happier with the results captured in this mode, rather than with stock processing. The HDR mode closes the gap between this camera and top-tier smartphone cameras, and it surprised me how many areas of processing are improved with it enabled. Of course, this comes at the expense of camera performance, though the Mi 6 isn't overly slow with HDR enabled.
HDR shot
Then there's the zoom camera. You should be aware that even though the toggle between 1x and 2x zoom is always available in the camera interface, the secondary zoom camera is only used in good lighting. This means that if you're indoors (even in reasonable lighting) or in a darker environment, the 2x zoom toggle will switch to using a digital zoom on the wide camera. Naturally this leads to a serious reduction in detail and generally rubbish results. There's no way to modify this behavior to force the 2x mode to always use the zoom camera.
HDR shot
Oddly, though, I was more impressed with results from the 2x zoom camera than the wide angle camera in general. Processing between each sensor is slightly different, with the zoom camera exhibiting better metering - images are a warmer, more natural tone - plus superior dynamic range and marginally improved fine detail. Of course you also get the added benefit of the more narrow field of view, which helps create a stronger depth effect for your macro shots. Don't expect to use this camera to magically see distant objects in great levels of detail, though, as 2x zoom isn't that much zoom.
I wasn't hugely impressed with Xiaomi's camera app, which hides a lot of functions underneath the 'mode' selector, including the settings screen. There's an okay manual mode included, along with a few other somewhat interesting shooting modes like tilt shift. Arguably the best feature is the portrait mode, imitating the same functionality on the iPhone by providing simulated bokeh for your images. When it works, it produces great photos. When it doesn't work, the fakeness of the effect is very noticeable.
HDR shot
Presumably to save component costs, the Mi 6 is a flagship camera that doesn't place much emphasis on improving focus or capture times, instead relying on traditional phase detection autofocus rather than having dual focus pixels or laser-assisted autofocus. As I said earlier, the Mi 6 isn't slow, but it's not as fast as cameras that have either of those features.