I think the lens was directed towards the Canon 1.6x format.
If you really wish to constrain the image circle further and as a result, make the lens render the transition image-black void much more distinctly, one can always leave the inner detachable half of the lens cap in place. The circle projected is cut off sharply due to the immense depth of field. There is nothing left of the blue-fringed transitional zone so characteristic for fisheyes.