More like there is this rather fuzzy peripheral part of the image that doesn't contribute much to the overall impact. Also might lead to cutting off the image circle, so adding the ring that comes with the lens (acts as a protection, and a mounting platform for a lens cap) is recommended.
Those Entaniya fisheyes are quite intriguing, I'll have a look at the product range after the upcoming move out of Oslo that hopefully does double duty by improving my economical situation