The Samsung Galaxy S8 rear camera has really good optics, but I wasn't pleased with the the image quality in the corners of the frame. I've gone over many sample images and observed the image quality (mainly the sharpness) in the edges of the frame. Usually when you look at the center or when observing an image at a small scale, it's really hard to notice any imperfections. Those rear cameras have a wide-angle lens. Some manufacturers use a lens construction with less lens elements.
I've checked out the camera exact specifications, and the S8 rear camera features a 6P lens. Just so you know, the secondary camera module features a 5P lens. With more lens elements, the lens manufacturer can optimize the light transmission path and making sure it's properly aligned. Without proper alignment, light coming from the outer region of the lens will appear distorted. We usually see this on cheap smartphone. The Samsung Galaxy S8 is a premium smartphone, and its camera module is designed for the highest performance, both in terms of image processor, image sensor capability and of course, the optics.
In the Galaxy S8 camera samples that I've seen, I've noticed that the corners of the frame, especially on the left side, were really blurry. There is also significant curvilinear pincushion distortion, but it's more noticeable when shooting pictures of objects with straight lines like buildings. You can check out the high-res sample images on ephotozine.com and see this for yourself.
Just keep in mind that many smartphones cameras suffer from the same thing, but I was expecting a better performance from the S8 in that regard. I don't know if the Galaxy S8 main camera uses aspherical lens elements within the lens construction. I know that the Huawei P10 Plus rear camera does because it uses a Leica Summilux-H 1:1.8/27 ASPH. lens (ASPH. is short for Aspherical). I've checked out some sample images taken with the P10 Plus and its primary camera produces much sharper results in the edges of the frame and with less optical distortions. I personally was more worried about the sharpness, because it was really blurry on the S8. That being said, most people won't even pay attention to this. We usually view the image while focusing on the center of the frame where the subject usually is. Even if the subject isn't at the most center area, it will still appear sharp.
Things get more blurred when you look at the edges of the frame. For some photographers this might be a warning signal because they might prefer a camera with better edges sharpness performance than what the S8 offers. This is one reason why many professional photographers use DSLR cameras with large lenses, because many of those lenses offer significantly better optical performance compared to mobile phone cameras. For example, you have the Nikon 14mm f/2.8D and Canon EF 14mm f/2.8L II USM which are both rectilinear lenses. They can shoot at a wide-angle without bending.
Also keep in mind that in the many cameras sample that I've seen, the left edges are more blurry than the right edges for some reason. I've checked many S8 samples just to make sure it isn't a defect in the specific S8 model they reviewer used. I don't expect perfect performance from a mobile phone camera's optics, but some of you might want to compare the edges of the frame compared to the Huawei P10 Plus and see the differences. I did, and this is why I learned to appreciate that Leica optics even more. So there are mobile phone cameras out there that can outperform the Samsung Galaxy S8 primary camera in that particular area. If you care about the optical performance in this specific area, you might want to check out the P10 Plus and compare the two yourself.