It was purely by chance that I was reading a thread on www.dpreview.com about fisheye lenses when I came across a message from "kli" telling me something that was to make my day - the following photos speak for themselves.
Sigma 8mm in its "normal" state
Here's the, unknown until now, hidden "collar"
Here's the Sigma fisheye in all its naked glory!
Sigma 8mm with "hood" on!
Sigma 8mm with "hood" removed!!!!
Image defished using "RectFish"
Defished image after CS2 colour-correction and a little cropping
I had a look at this lens in a photo store and the salesman also hadn't realised that the collar came off!. It seems that the collar is necessary to allow the lens cap to be retained without touching the front glass. Not sure but it could also be used to support a filter if you accepted the vignetting in the image. I am thinking of buying this lens but will wait till I see the new 4.5mm sigma.