Both the rear and front cameras of the Samsung Galaxy S8 feature a F1.7 aperture lens. We usually get to see only one of the cameras having a fast aperture, usually the main one. This time around, Samsung wanted to make sure that both cameras can perform well in low-light. The rear camera has an advantage of having larger pixels (1.4µm vs 1.22µm), a larger sensor and an optical image stabilization that helps improve its low-light performance even more.
For those of you who aren't that tech savvy. The f-number signifies the hole through which light can pass through the lens. The larger that hole, the more light which can pass through it. Now, the smaller the f-number is, the larger this hole (aperture) is. So we want this number to be smaller if we care about low-light performance. You've probably seen cameras that come with f/2.2 or f/2.4 aperture for example. So because this number is smaller than f/1.7, this means that it transmit less light. By the way, you can right the aperture with or without the slash, like so: either F1.7 or f/1.7 with a small or large F letter, it doesn't matter.
Now, to calculate how much light can an F1.7 transmit (exposure) compared to another lens, I recommend using this online 'How many stops?' calculator. So let's compare an F1.7 aperture to an F2.2 lens. Just make sure that you put the same shutter speed value in both the first and second settings so you get the difference right based on the aperture only, not the shutter speed (I put 1/200 in the shutter speed in both fields). Now according to this calculator, the different between the two is 2/3-stop and in more precise stops it's 0.7439. You can see that the exposure ratio upper:lower is 1:1.675, which means that F1.7 aperture results in approx. 1.7 times more light compared to an f/2.2 lens. In general terms, 1-stop equals to two times more light. For example, the difference between f/2.8 and f/4 is 1-stop.
It might be counter intuitive to think that a smaller f-number means something more, but that how it works with lens apertures, so just embrace it. A smaller aperture value also affects the depth of field. If you take two cameras with the same sensor size, same focal length and you shoot a subject from the same distance, a the camera with the larger aperture (smaller f-number) will produce an image with a more prominent depth of field effect, because the area of focus is narrower. In mobile phone cameras you probably only notice it when shooting macro (close-up) shots. It's really isn't a big deal, especially with current technologies that produce fake bokeh with pretty convincing results, something even the fastest mobile phone camera lens can't give you, only a professional DSLR camera with a large image sensor and a very fast lens.
Now, the most important thing to note here is that both cameras of the Galaxy S8 should perform better in low-light compared to a lens with a slower lens (a 'slower'l lens means a lens with a smaller aperture, with a higher f-number). An F1.7 is is usually used in upper mid-range and high-end smartphones and it's great to see Samsung bringing it to both the front camera as well. You can now take low-light selfies with more confidence, knowing that you are less restricted shooting self-portrait pictures when the sun fades away into the horizon. You can take selfies in indoor areas with very little light, yet still get a well exposed and usable image. The front camera lacks OIS, so you won't be able to get good images in very restrictive lighting conditions. The front camera might be able to get a good exposure by bumping up the ISO, but this will result in more image noise, something that you probably won't to avoid unless there is no other way to get that shot that really want. Oh, a faster aperture for the front camera will also help you get portrait shots with a more shallow depth of field effect, which is another great benefit for having a fast lens for the front camera.
Before ending this Advantage, let's see what aperture the other leading flagship smartphone have for their front and rear cameras.
Samsung Galaxy S7: f/1.7 for both rear and front cameras (same as the S8)
Sony Xperia XZ Premium: f/2.0 for both rear and front cameras (S8 is better, 1/3-stop faster, 1.39 times more light)
iPhone 7 Plus: f/1.8 rear camera, f/2.2 front camera
OnePlus 3T: f/2.0 for both rear and front cameras
Google Pixel XL: f/2.0 rear camera, f/2.4 front camera
LG G6: f/1.8 for for the normal angle rear camera, f/2.4 for the ultra wide-angle rear camera, f/2.2 for the front camera
Of course keep in mind that the aperture is not the only factor influencing the low-light performance, but it's an important one nevertheless.
So Dual F1.7 aperture for both the rear and the front is definitely a great advantage, especially for low-light. Happy shooting!