Answers to several questions:
"...The only thing to improve is the cyan halo on the border , is this some sort of chromatic aberration ?"
I'm not sure what caused that...or how to get rid of it. But I agree the circular shots would look better without it! If anybody has a solution, I'd love to hear it!
"I am relatively new to this fisheye shooting, can I ask how you did the first shot?"
Well, you need to have a fisheye lens, and for the circular shot, you need a circular fisheye lens like the Canon 8-15mm lens I used.
"Have been considering one of these but am wondering the benefits of the 8mm end, particularly as uptake of this lens increases and the uniqueness of the circular image is well and truely lost. The other benefits would appear to be better image quality and improved focusing compared to the Canon 15mm. Can you comment on that Bruce?"
I think I addressed the first part in my answer to Alex. I personally think people will actually get SICK of seeing circular images after awhile, so you really need to feel you can get something (special compositions) out of the 8mm end of the lens that "transcends" the fact that the images are circular. Frankly, I'd love to see a "rectilinear circular fisheye" lens

where you had the 180 degree view in all directions but a square or rectangular image (of course, that's never going to happen, since the terms themselves are contradictory)!
As for image quality and focusing, I can't compare quality to the Canon 15mmFE because I have a Sigma 15mmFE, but those two 15's are considered quite similar for image quality, and my SUBJECTIVE opinion is that these images are somewhat sharper than my Sigma (which is no slouch).
As for minimum focusing distance, there's a definite advantage (the Canon 15mm is 7.9", whereas the Canon 8-15mm is 6.2"). Whether that's worth the extra money, I can't say. If all you want is closer focusing distance, you could just switch to the Sigma 15mm, which is 5.9" and a heck of lot cheaper than the 81-15mm.
Bottom line: I feel like this lens gives me a combination of benefits that add up to it being worthwhile: higher quality than my Sigma 15mm for regular fisheye images, close focus, more solid "L" quality build, and the flexibility of a zoom lens, specifically the
option to use the circular fisheye (8mm) when I think it might help create a unique image. BTW, I sold my Canon 14mmL II lens, which was "OK" underwater, i.e., not discernably better than my Sigma 15mm, and the proceeds were slightly more than what I paid for this new 8-15mm lens. I feel like that was a definite "step up" for my underwater photography kit.
Hope that helps!
Edited by bmyates, 24 February 2012 - 10:43 AM.