Hmm OK.. since I do not have this lens.. I better stop saying anything about the Canon Fisheye 15mm.
Yesterday we were talking about a straight NON Fisheye 15mm Sigma and Smooth sorted it out. Said it would not work with PW4.
Nuf said
But here is a link to the Sigma 15mm Fisheye lens that does provide a diagonal 180 degree angle of view. Maybe it has something to do with the Nikon d70, Canon 350d and 20d crop factor of 1.5 - 1.6
http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3279&navigator=4
The Nikkor 10.5 seems to work ok 10.5 x 1.5 = 15.75mm
The Sigma 15mm x 1.5 = 22.5mm
The Sigma 8mm on a Canon 5d full sensor is in fact 8mm full circle.
/s/
Dave
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