ya har. although MOS is particularly wide. hence the Tokina fisheye... 10mm on that is actually about 9.4mm or something where as 10mm rectilinear is nearer 10mm...
I know this is not a technical thread, but a fish-eye lens has a completely different design from a rectilinear lens, which explains the difference in angular field. A 9.4mm rectilinear would only be a little wider than a 10.0mm, whereas on a crop camera the rectilinear 10mm has about 108 degrees and the fish-eye almost 180 degrees. The central part of the image is the same size for both, but angles are reproduced progressively differently as you go out towards the edges.





