The Galaxy S8 has its fingerprint reader in a very uncomfortable place to reach if you use the phone with your left hand. However, the main problem with its position is that it is placed near the rear camera lens. People who try to unlock their phone or use the fingerprint reader for authentication in apps, can accidentally put their finger on the camera's lens. If your finger is not very clean, the glass will collect smudges and dirt and this can have a negative effect on image quality. The reason for that is the light that passes through the dirt will be shifted and that can lead to undesirable optical effects.
When people mentioned it in forums and reviews, I actually didn't know how bad this can be and how exactly it affects the image quality. However, this issue happened to me a few days ago on my trip to Porto in Portugal. I was visiting a beautiful book store called Livraria Lello, also known as The Lello & Brother Bookstore or Livraria Chardon. It's a stunning bookstore located in the heart or Porto. OK, I paid 4 euros and went inside. I was so stunned by the stunning magical Gothic design that I took my iPhone and start taking pictures. I was shooting like mad because this place was so unique and special and I really wanted to share those sights with my family back home.
After a few minutes I took a moment to look at the photos. I've notices that many of the photos lack contrast and there were bright light rays coming from places where there is a strong light source, were the lamps were positioned. Although I like this effect on some of the images, it wasn't desired in most images. The reason for the strokes of light is because the light is reflected inside the lens to different directions. This causes some glare effects and it considerably reduced image clarity and contrast.
I first didn't know what the reason for that, but I've decided to clean the lens of the camera with my shirt. I took a few pictures again and all of them looked great.The same thing can happen with the Samsung Galaxy S8 rear-facing camera or in any phone for that matter. But with the S8 it's more likely to happen if you occasionally use the fingerprint reader for authorizing apps and unlocking the phone with it. There is a high chance that you just will accidentally put your hands on the camera's front glass. If you do this when you are outside and your hands a even a bit dirty or have oil residues, those can stuck on the glass. You won't notice it until you view the images. The image quality can be negatively affected by this and it can look even worse then the what you see in my images.
So it's not like reviewers enjoy complaining about that weird positioning of the fingerprint reader, it's really a terrible place to put it, for both inconvenience and for the reason I've stated above regarding dirty and oil residues that can stay on the camera's front glass. This is certainly a design disadvantage that has a high chance of affecting the image quality of the photos that you take with your Samsung Galaxy S8 rear-facing camera. Dirty optics are just a no-no in photography. To get the best image quality you need to make sure that your Galaxy S8 lens is clear of any dirty, oil residues, dust, etc. Samsung even seems to acknowledge it by notifying the user to make sure the lens is clean before taking a shot. Anyways, bottom line is that this is an issue and I hope that next time Samsung will reconsider where it puts the fingerprint recognition module, and you, just make sure the lens is clean before you take a series of shots. Happy shooting!