Both phones sport a 12-megapixel rear camera, auto-HDR, have exposure controls, and have a 5X digital zoom.
But the iPhone 6s has an f/2.2 aperture while the iPhone 8 has an f/1.8 aperture. This means the iPhone 8's camera is able to let in more light, making for better shots in low light and sharper images overall.
The iPhone 8 also has optical image stabilization (OIS), a six-element lens (compared to the iPhone 6s' five-element lens), wide color capture, and a quad-LED True Tone flash with slow sync.
I primarily focused on testing the phone's abilities in low light, since that was the biggest change from the iPhone 6s.