One of the key features and main advantages that the Xiaomi Mi 6 brings is a dual-camera system. A single-lens camera offers a fixed focal length (no zoom or 1x zoom). However, the Xiaomi Mi 6 features two 12MP cameras on the back, one is a wide angle and the second is a telephoto lens. Users can seamlessly move from wide-angle to telephoto with a single tap. Xiaomi called this 2X optical lossless zoom, and it's indeed lossless, because there is not digital enlargement involved. The Mi 6 does suppose 10x digital zoom with ISP algorithm, but the digital zoom is lossy, which means that there is a negative impact on the image quality, because the image is just enlarged and cropped.
Some companies use dual cameras for different purposes. For example, the Dual camera of the ZTE Blade V8 mini features a 13MP RGB sensor with a 2MP depth-sensing sensor for a more artistic and blurry shallow depth of field effect. The Oppo F3 Plus dual-camera system features a first lens with 84-degree FOV, and a second lens with 120-degree FOV (ultra-wide angle). So there are plenty of uses of which phone manufacturers can benefit by using two cameras at either the back or the front of the phone to enhance image quality and improved its versatility.
I personally prefer an ultra-wide angle lens because I usually shoot in wide-angle, but having a second lens with a higher focal length like in the Xiaomi Mi 6 has its benefits as well. By using the second lens, you can get close to the subject. The Mi 6 dual-camera also uses some software tricks to produce artificial shallow depth of field effect. There is a portrait mode in which the camera can blur the background and get rid of distracting background elements.
By the way, this is the same tech used in the iPhone 7 Plus. The iPhone 7+ also comes with two cameras at the back, one is a "normal" 28mm-equivalent (in 35mm terms) lens and the second is a 56mm-equivalent telephoto lens. The Xiaomi Mi 6 zoom dual camera features a 27mm equivalent focal length lens and a second 52mm equivalent focal length lens. By the way, from my understanding based on the official Mi 6 camera specifications, the OIS only works for the first wide-angle camera, not the telephoto camera.
The thing is that you get to enjoy the same great technology but in a smartphone that costs as low as CNY 2,499 ($360, for the 64GB version). This is amazing to see this technology in a mid-range phone. I knew that in 2017 many cameras will use the dual-camera technology and we are going to see it penetrating the mid-range and even the upper low-end market as well. According to one of the slides posted on Xiaomi's twitter, the Mi 6 uses a Binocular imaging system with deep learning algorithm, which helps to clearly define area of focus and blur background in more precise manner. We've already seen that the simulated Bokeh effect isn't too accurate from what we've seen on many phones with a dual-camera setup. So Xiaomi took it a step further and added advanced algorithms algorithms that will help to improve the results to provide more accurate subject isolation, so even hair can be detected so the blury effect can be applied properly in that area as well.
Oh, one last thing, did you know that according toe Xiaomi, the Mi6 is the first 5.15" smartphone with dual camera, just so you know.