Histograms for each of the five images. This (together with the viewfinder's manual exposure over/under meter) is how I determined how far to rotate the filter ring for each stop. I attempted to keep these constant. My camera shows only the green channel and you can see from these histograms that the green channel is basically constant through the first 4 stops. Beyond the 4th stop, the color shifts of the red & blue channels begin to dominate the histogram but the overall exposure is constant throughout all 5 images.

When I set the camera's exposure to 8 stops above the "no filter base" the histogram values were compressed entirely within the upper 50% and climbing the right wall - that is, totally overexposed. Conclusion - filter can't allow 8 exposure stops of light to pass through the lens contrary to the advertised claim.