I had a play with one of these on a 5D3 the other day, and came to the conclusion that it's the same focal length at both ends.
What do I mean?
Well, I looked at the edges when it was at 8mm and in fish-eye mode, and then while at 15mm and the same area was covered i.e. a table was at the edge of the frame
Nice lens, but the fish-eye end just seems a bit of a gimmick.
Yes. It is pretty much the same degree fov. But that does not make them one and the same.
There is no "fish-eye end" since it is all technically fisheye across the whole range.
On a 5D3, you get a circular fisheye at 8 and a regular fisheye at 15. Two completely different things. Not a gimmick.
Lastly, the main motivation for this lens was to have one fisheye to rule them all with regard to providing a true fisheye with all sensors in one lens. This means all three of canon's sensors, if you happen to own them all have fisheye capability with it unlike any other fisheye on the market today (full frame, 1.3x, and 1.6x).
All zoom lenses work this way.., to some degree. The lens makes an image circle, and while you zoom, the circle gets bigger and smaller. In the fixed size of the image sensor, the image zooms in and out. This is why some lenses went for crop sensors can cover the whole frame in FF. I have a tokina 11-17, and it works great on FF but only at 15-17.
I am not a huge fan of the circular fisheye image, and I rarely use the lens this way. But the zoom is very useful for 2 other reason. On my 1d4, and 7D, I can zoom to the most fisheye, and the least fish that still cover the frame. If I know i am shooting a panoramic or a square cropped photo and zoom as appropriate for that look even on my 5D3.