For istance what happens to say a 15mm fisheye or a 8mm circular fish eye? do they simply become a 22.5mm fisheye and 12mm circular fisheye? Or does the multiplier screw that up. I mean I've never heard of a 22mm fisheye
Well, yes, sort of. JRP's comment is correct but incomplete. A fisheye becomes a non-rectilinear corrected wide angle lens. With your rectilinear wide angle lenses, you can essentially just consider the lens to be multiplied by 1.5x so a 14mm lens would have the same field of view as a 21mm lens would on a 35mm camera. Note that the perspective doesn't change from that of a 14mm lens, just the field of view. This is farely noticable with the wide angle lenses. With the fisheyes this especially holds true. So in essense, yes a 16mm fisheye becomes a 24mm fisheye with a totally different look from a 24mm lens. I've attached the crop from the bottom edge of a photo taken with the 16mm Nikor on the D100. Notice how the straight edge of the bar counter is heavily curved, looking almost circular.
DaveDosch Take nothing but pictures, leave nothing but bubbles.
Well, again, almost. The nature of a fisheye is the distortion is more apparent farther away from the image center.
Here's a 16mm Nikkor fisheye shot and the same shot with the 1.5 crop. If you backed up far enough to cover the same amount of the foreground subject, this area would be coming out of a less distorted area of the image projected onto the imaging plane.
So while an effective 24mm non-retilinear lens is heavily distorted, it's not a fisheye anymore.
With a true circular 180 degree angle of view fisheye, the crop would lop the top and bottom off the image circle for a shot made with the camera held horizontally.
To visualize why a crop is less distorted, imagine a camera with a 20X crop factor. When you trim away the extreme distortion of the outside of a fisheye image, the center is only modestly distorted since the fisheye distortion is a spherical projection.
Of course if you want a fisheye effect you can use the spherize effect in Photoshop to create one. Just be sure to make your canvas square if you don't want an eliptical distortion...