It’s the first full-range fisheye zoom made for APS-C reflex cameras.
On older version f/stops are going from 3.5 to 22, while on new ones from 3.5 to 16. The mirrorless version is the same lens with a tube attatched to match the correct flange distance. This always means a front-heavy lens.
On an APS-c sensor, the zoom range allows to go from circular to diagonal fisheye, as it was an 8-15mm on a full-frame sensor.