The Nokia 6 comes with a fast f/2.0 aperture lens. I know that many enthusiast photographers and Nokia fans would probably be more happy with a f/1.7 lens, it's still a fast enough lens to help taking good photos in low-light, especially considering its very small 1-micron size pixels. Furthermore, unlike some previous Nokia phones, this one doesn't come with a Carl Zeiss optics. There isn't any mention of the glass manufacturer, and I believe that Nokia is saving the best for last, and we'll get to see some Zeiss optics in its higher-end models.
Just for comparison, the Samsung Galaxy S7 features a F1.7 aperture lens, the Galaxy S6 comes with a maximum aperture of F1.9, HTC 10 has a F1.8 lens and the iPhone 7 comes with a bright F1.8 lens. We can see that most of the high-end devices enjoy a very fast aperture, which is less than f/2.0. f/2.0 aperture is more common in mid-range devices.
So what is the difference between f/1.7, f/1.8, f/1.9 and f/2.0?
Well, a f/1.7 aperture lens gives us 1/3-stop (0.4689 stops to be exact) advantage compare to a f/2.0 lens for the same given shutter speed. This results in an exposure ratio of 1:1.384. In other words, a f/2.0 lens, results in 1.4 times less light compared to a f/1.7 lens and that words the other way around of course. A f/1.7 is "faster" than a f/2.0 and results in approx. 1.4 times more light. f/2.0 compared to f/1.8 means approx. 1.24 times less light and 1.1 times less light compared to a f/1.9 lens.
So a larger f-number means that the lens results in less exposure compared to smaller f-number. This is why the Nokia 6 rear camera, at least as far as the lens is concerned, is not as good as the ones offered by the Galaxy S7, iPhone 7 and the other phones that have a "faster" aperture lens, a smaller f-number.
1 stop of exposure equals to times the mount of light. The different between all this aperture f-numbers: f/1.4, f/2, f/2.8, f/4/ f/5.6, f/8 is exactly one stop, so each one-stop increment means results in half light gathering area compared to the previous one. This is why many photographers look for a mobile phone camera with the fastest lens. It doesn't necessarily means that the camera with the fastest lens will have the best low-light performance. There are other components involved in the overall low-light performance of the camera, including the sensor technology, pixel size, optical image stabilization, image processing, etc.
So a F2.0 maximum aperture, it's is the maximum because you can't get a "faster" aperture than that, is relatively fast in photography terms. In mobile phone cameras we got used to fast apertures, but if you buy an interchangeable lens camera like a mirrorless camera or a DSLR camera, those what called "fast" lenses, can be really expensive, depends on the model.
So if someone asks you what is better for a mobile phone camera, a F2.0, F2.2 or F1.8, you know the answer. A faster aperture lens also results in a shallower depth of field effect. The depth of field is influenced by the distance of the sensor from the subject, the aperture and the focal length of the lens. So the lower the aperture is, the more limited the depth of field is, which helps to produce a large area which is out of focus, an area that resides outside the DOF, the area that it is in focus. This is another reason, and probably one of the main reasons why professional photographers prefer working with faster lenses, because the blur effect is much more prominent.
This also works with the Nokia 6 when shooting close up objects, but just keep in mind that you have to be close to the subject to see this effect because of the smaller sensor.
So the f/2.0 lens of the Nokia 6 does help in low-light, but if it had large pixels, it would have produced much better images in low-light. It's a compromise that Nokia obviously knows about, and in this specific model, Nokia thought that it's better to give users to enjoy a higher resolution image than using a sensor with larger pixels and a lower resolution one. Overall, a good fast lens that can get you some good low-light shots, but unto a certain degree. The Bokeh effect should be quite good overall.
I know that Nokia fans expect Nokia to be the king of image quality, low-light performance and camera innovation. I think Nokia will surprise us, but not in this specific model, the Nokia 6.