Fisheyes should be shaved

I have been an amateur photographer since more than 30 years - but it took me 28 years to come to a fisheye lens.
Both, the very spectacular perspective and the expected low cost-benefit ratio seemed, the expenses are not justified.
Now, from the viewpoint of Panorama Photography, things changed completely. While prices for Fisheyes dropped a lot, those lenses are really great for Panorama Photography - and highly valuable.

Meanwhile, all the well-known camera manufacturers do offer at least one fisheye lens in their product portfolio. They are focusing on the full-frame fisheyes with a (diagonal) FoV of 180 degrees. On a full-format DSLR, the related focal length is about 16mm, and some 10mm and 8mm for the APS-C/DX-format and MFT, respectively.

Typical repressentatives of this kind of lenses are the AF Nikkor 16mm 1:2,8D, the AF Nikkor 10,5mm 1:2,8G ED, the Canon EF 15mm 1:2,8 or the Olympus Zuiko 8mm fisheye. Sigma as third-party is now also shipping a 10mm F2.8 EX DC full-frame fisheye.

All of those lenses is common, that the diameter of the image circle is equivalent to the diagonal of the sensor of that type of camera, they are designed for. The image circle is cut from the horizontal and vertical edges of the sensor and shaded from the four little stumps of lens hood. The shadowing you can only realize, once you try to utilize the lens dedicated for APS-C/DX-format on a full-format camera, e.g. a 10.5 mm DX Nikkor on a D3 or D700 body and disable the DX lens detection flag.
Doing so, you come just strait up with the idea to remove the mutilated lens hood.
Some freaks have maltreated the lens hood using jigsaw and carpet knifes. Well, I asked my nephew to remove the plastic stumps on the turning lathe. However you manage to to get rid of those excrescent plasic parts, the desired positive result will be the same in any case:  the full image circle of your lens, that was only visible in the diagonal of the sensor, is now usable on the full-format sensor in all it's beauty - and pin-sharp ! 

The result in full-format can be seen !
The image circle of the so-called portrait fisheye with a horizontal FoV of at least 180 degrees and a vertical FoV of some 140 degrees, is only limited by the narrow edges of the sensor. Rotating the camera in portrait orientation around the no-parallax-point, you can generate a full spheric panorama with only 3 shots. Look here, how it looks like.

Surprisingly, third-party-vendors have not yet identified that market gap. It seems as if there is no room for something in between a circular and a full-frame fisheye. For instance, Sigma has been marketing the well-known 8mm F3.5 EX DG Fisheye as circular fisheye for full-format DSLR's while it is the real classical portrait fisheye on APS-C/DX format DSLRs, and they are promoting the new 10mm F2.8 EX DC fisheye as a full-frame for APS-C/DX format, while it is really fantastic panorama lens for full-format DSLRs. But there are some more, for instance the affordable Samyang 8mm or the well-known Tokina 10-17mm.

The only thing you need is a jigsaw - or the professional shaving service: Tobias

update Feb 8, 2010

I got the chance to have a first look on the Samyang 8mm Fisheye.
It seems to be a very interesting lens. Hope, to get one for some shootings soon.
Have a look here, too:  Samyang 8mm  (by Michel Thoby)