Originally posted by AfterPentax Mark II Quite often a fisheye lens for an APS-C lens is described as a diagonal fisheye.
...as opposed to a circular fisheye where the image circle describes 180° and is smaller than the frame. The term "diagonal" just means that that the field of view is 180° on the frame diagonal and that the image fills the frame, regardless of frame size. Both terms go way back.
As for other than APS-C, my KMZ Zenitar 16/2.8 Fisheye is a diagonal fisheye for the 24x36FF format as is the highly respected Sigma 15/2.8 EX DG Diagonal Fisheye. Pentax does not currently offer a fisheye prime, but both the Pentax-K 17/4 Fish-Eye and Pentax-A 16/2.8 Fish-Eye are also diagonal fisheyes along with the Fish-Eye Takumar 17/4, S-M-C Fish-Eye Takumar in M42 mount and three named variants of the 35/4.5 fisheye for the Pentax 6x7/67. I don't believe that Pentax ever made a circular fisheye.
Steve