One of the main highlights of this phone resides at the back with dual cameras offering. One cameras features a 12MP sensor with a wide-angle camera lens and the second camera is a 2x optical zoom telephoto lens. The second camera doesn't zoom and when I say 2x optical zoom, I just mean that the focal length of the second lens is twice (rounded up) of the first lens. The first wide-angle lens has a 26mm equivalent focal length with an f/2.2 aperture lens and the second camera uses a 50mm equivalent f/2.6 aperture lens. The 2x represents the division of the highest focal length number (50) by the lowest focal length number (26), rounded up.
This is roughly the same setup that we've seen other phones like the Samsung Galaxy 8 2x optical zoom, the Oppo R11, the Asus ZenFone 3, and Apple iPhone 7 Plus among others. Some phones use an ultra wide-angle lens instead of a zoomed one, which is different of course. With an ultra wide-angle lens you can take pictures with much wider field of view (FOV), whether with an optical zoom lens like the one used in the Xiaomi Mi A1 (5X) you can take pictures with a narrower field of view.
So what are the benefits of having a 2x optical zoom lens you ask? well, you can take closeup pictures and get closer to the subject without moving. The lens will magnify the subject and it will appear larger in the image, all that without any negative impact on image quality. This is unlike digital zoom that just digitally enlarges the image and crop it, which has a negative impact on the image quality and should be avoided. There is always an option to enlarge an image ins photo editing apps and it's better to shoot a regular image which has more details in it.
The Mi A1 dual camera is a great feature that until recently could only be found in flagship smartphones. Nowadays we can see more and more phone manufacturers bringing this dual-camera setup to mid-range devices. I'm sure that next year we are going to find those in some entry-level devices as well, but maybe with a less impressive specifications.
2x Optical Zoom is especially useful for shooting portraits and this is where this camera really shines. If you ever wanted to buy a smartphone with dual cameras and couldn't afford buying one, this is your chance with the new Xiaomi Mi A1 (5X in China).