In article <58lfmn$nvf@tivoli.tivoli.com>, bowman@tivoli.com says...

>Just out of curiousity, some short focal length lenses are labeled fisheye and
>other are not. Is there something unique about a fisheye other than the short
>focal length?

Yes, the perspective type rendered. Most lenses are designed to render
subject straight lines as straight lines in the image (though few lenses
50mm and under actually do more than approximate this...), and are called
"rectangular perspective" lenses. There are other perspective types, the
most common of which are "spherical" (looks like the rectangular-type image
projected onto the surface of a sphere - straight lines running through
the center remain straight, but ones off center are progressively more
curved with greater distance from the image center), and "cylindrical"
(looks like a rectangular-type image projected onto the surface of a
cylinder - straight lines [usually the verticals] running along the cylinder
remain straight, but the straight lines running in other directions are
mostly curved). Fisheye lenses have spherical perspective, rotating-lens
panoramic cameras have cylindrical perspective.
Hope This Helps