basically the camera is "immersed" in a glass cube size 10x10x1 cm with specular material assigned.
Playing with the index and the focal length, you can play almost faithfully the Fisheye effect with super acceptable rendering times. In the first tests I have played a bit with the value of reflection and activating and deactivating the dispersion that, in the end, I decided to keep active because chromatic aberration, which normally i hate, in this case, helps a lot to describe the effect.
here is the results, pure octane output, no post:














ciao beppe