With a Canon APS-C body you can go linear-ultrawide
with the Sigma 10-20mm, the Tamron 11-18mm, Tokina 12-24mm and the Canon EF-S 10-22mm f/3.5-4.5 USM
with the Tokina 10-17mm or the Sigma 8mm
With Nikon all the third-party lenses work ok, but it also has the 10.5mm fisheye and the linear 12-24 zoom.
If Nikon had a linear 10-22mm I would have bought it instead of my fisheye.
So if you want to go linear-ultrawide-original Canon is in a better position than Nikon. If you want to go fishy-ultrawide-original Nikon is in the lead.
Sony has the linear AF 11-18mm 4.5-5.6 DT and no fisheye
Oly has the diagonal-fishy Zuiko digital ED 8mm 3.5 plus the linear Zuiko digital ED 7-14mm 4.0. Quite a feat!
Pentax has the smc DA 10-17mm 3.5-4.5 ED Fisheyezoom plus the 12-24mm linear
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