... and for those who experience ill effects from "rolling ball" the distortion follows the mustache pattern of pincushion gradually increasing out to about 70% of the FOV, then quickly reversing until there is no pincushion and very high angular magnification distortion near the edge. I'll put up photos in the review.
It seems like this is the modern trend for wide field binoculars. I've noticed this pattern slight pincushion in the middle reversing to high AMD at the edges on (just naming some binoculars I've owned recently) the Kowa Genesis 10x33, Kowa BDII 6.5x32, Nikon Monarch HG 8x42, and to a lesser extent the Nikon EDG 10x32 and Zeiss Conquest HD 10x32. I also have a Leica UVHD 8x32 which has zero AMD, just pincushion and slight field curvature, so I have a frame of comparison.
Is this just an "easier" way of achieving a wide FOV with a reasonably sized ocular? It feels almost like "cheating" in a way, advertising a wider TFOV but then underperforming in AFOV because the extra FOV is just squeezed into the edges.
Of those I've owned above, the EDG in my estimation has the best overall "balance" of distortion where the field feels natural while panning. The Kowas showed the most AMD / compression and felt the least natural panning.
One thing which seems to be a differentiator in how "natural" the looks appears is point at which the "compression" starts (which is I assume where the transition / inflection between pincushion and AMD begins?). For example, when doing the test of moving a small circular object from the center towards the edge of the FOV, the Kowas showed visible compression/deformation very early, it was visible under 50% from the center axis. And the compression at the edges is quite severe. The EDG on the other hand has slight pincushion but significantly less compression until you reach the other 10-20% of the FOV.
The degree to which the compression encroaches into the center of the FOV appears to me, based on my recent experience with these models, to be highly correlated with the visibility of "rolling ball" when panning. The worst offenders give a "funhouse mirror" effect where space ripples and deforms, especially obvious if you change direction (especially slight movements side to side, like when tracking a small bird in a bush that is flitting around a lot).