I've checked the Nokia 8 vs LG G6 and Galaxy S8 in gsmarena photo compare tool and you can clearly see that the Nokia 8 doesn't match up with the G6 and S8 when it comes to lens sharpness. As you probably know, the Nokia 8 uses Carl Zeiss optics. I've seen many sample images and actually found the sharpness to be very good. The thing is that until you compare the image quality versus other phones, you don't really get to see how good it is. If you visit this comparison page on gsmarena, you can clearly see that the Nokia 8 rear camera is considerably less sharp than the LG G6 and Galaxy S8 rear cameras.
To see what I mean, point the cursor to where the currency bills are in the center of the image, slightly to the bottom left side. This is a good part to compare the sharpness of the lens because their is a high amount of fine details. You can clearly see that in comparison to the G6 and S8, the Nokia 8 image look quite blurry, compared to the S8 and G6 that resolve texture details much better and you can clearly see that the fine details are much clearer in comparison to the Nokia 8 camera sample. When looking at the 5 Dollas bill, the Galaxy S8 looks the sharpest of the three (take a look at Abraham Lincoln's face. In the Nokia 8 image you can see the fine drawing lines on the face at all, on the G6 you can see it near the cheeks and in the S8 you can see it clearly all over the face, including the forehead, cheekbones, mouth and nose. Even the eagle details are much more detailed. In the Nokia 8 test picture it looks almost flat whether in the S8 it looks much more detailed.
So it's a Zeiss lens at the back but the sharpness just isn't on par with the competition. I guess it's also the combination of a better sensor and more advanced image processor that helps the S8 outperform the Nokia 8 by such a large margin. That being said, optics are optics, and i think that the Nokia 8 should have bestowed a sharper image. I don't know which lenses the Samsung Galaxy S8 and LG G6 use, maybe Largan Optics. I also don't know whether being a Zeiss lens results in such a huge difference compared to less popular brand, because the mobile camera lens is much smaller compared to large interchangeable lenses. What I mean to say that it's not too expensive to make a high-quality camera phone lens compared to a DSLR, therefore the differences between a premium brand and less popular one are ineligible.
Judging by the studio image quality comparison, I have to say that I am disappointed with what I've seen. In regular sample images, before comparing it to other cameras, I was very pleased with what I've seen. But when you compare the Nokia 8 rear camera image quality to other phones in terms of sharpness/texture details, you can see that it's not that impressive at all, regardless of it carrying the Zeiss brand for its rear-facing camera.
You might ask yourself? Is it really bad, does it really make the image look blurry? My answer is of course not. In fact, this slight blurry results compared to the other cameras is visible when viewing the image at 1:1 scale in 100% magnification. With slightly reduced resolution you won't even notice any difference and probably won't see the difference in the fine details anyways. Most of us share images at much lower resolution, whether between each other via WhatsApp, Facebook, Twitter and son on. Most people don't do this extensive analyzing.
That being said, some people do care about it, and I think it's important to bring it up. Keep in mind that results may vary and maybe an update can improve what we've seen here. This is just an early assessment based on what I've see via gsmarena photo image quality comparison tool, which is a valuable tool because it allows you to compare camera samples taken in the same environment and of the same subjects.