So I had a gift card and was able to pick up the new 7Artisans 6mm fisheye for APS-C. It's a 220 degree diagonal fisheye, so...like a normal fisheye but even more extreme. And yes, it can see behind the front of the lens in the corners. As a result, you have to be extremely careful about how you hold the camera (a normal grip puts your pinky in the image frame), and even how you hold your arms (must tuck in to the sides, or again, your forearm will be in the image).
While this leads to some interesting things, such as being able to see the floor in all four corners on the first sample shot below, despite pointing the camera straight up and being 2 feet off the ground, the biggest impact to the extra width is that the horizontal field of view is very close to 180 degrees - so you have the ability to capture entire rooms, albeit with extreme fisheye distortion.
Obviously such a lens isn't going to get a ton of use, but for the $200 price, it seems to be quite a decent lens. It's f/2, and it's sharp in the center at that aperture, though the edges are a mix of out of focus and soft, due to the AOV. But it focuses extremely closely, so you can get some wild closeups like the second shot below (taken at f/2). Stopping down to f/5.6 gets it reasonably sharp across the frame, and f/8 and f/11 are even a little better into the corners. The extreme corners never get super sharp, but they get sharp enough, and the edges are pretty good at smaller apertures, so ultimately it produces good images for the price. For such an extreme lens, I'm quite pleased.
First, the lens on my Z50 II - it's got a good sized front element (though, just a WEEE bit smaller than the gargantuan Nikon 6mm f/2.8 fisheye from back in the day):
And some sample images: Most of these are at f/8, save for the second shot and 5th shot, which are both at f/2. Bokeh is pretty decent to be honest (at least in the background), though you'll only see it on closeups.
