In article <46m9jh$i6j@nntp.ucs.ubc.ca>, peter@ares.ubc.ca says...
>anyone out there ever used something called a "fish-eye" adaptor?
>I just read somewhere that they used to be fairly popular a few years
>back and they just attach to the front of your lens and act like a
>fish-eye. They have their independent aperature settings and have a
>lens barrel and everything.

There are two types: the one you described (Kenko and Spiratone); and
one that also attaches to the front of the lens, but retains the use of (and the advantages of using) the prime lens aperture. The latter type
reduces the prime lens focal-length less, so they are useful for simulating a full frame type of fisheye lens instead of a full-circle
or partial-circle type of fisheye lens. None that I have tried comes
close to the optical quality of even the worst prime fisheye (though
the latter type works very well on video cameras), but for $100 or so, one may be well worth buying for occasional use. BTW, the Sigma 8mm f4 prime lens is intermittantly available new for about $400, and is
excellent if you want to get serious about fisheyes. I love the durn
things and own three fisheye lenses (plus a converter for the video camera), and I also use them with a 1.4x converter to get an additional couple of fisheye focal-lengths. And, before anyone complains bitterly about the aweful distortion of fisheyes, let me add that back in my teaching days I had fun demonstrating to students how we actually see
in fisheye (spherical) perspective (with proof, and a simple demo so that people could see the line curvature [or, at least be sure that it must exist] for themselves).
Hope this helps.