I recently returned from a couple of weeks diving in Phili and tried out these two lenses on both my full frame ( 5D2 ) and creopped sensor ( 50D ) Canons in Subal housings. I was very pleased with both.
Quite a lot has been said here about the 8-15mm so I will just concur that it is quite stellar on both formats and certainly worth the extra $ in my opinion. Some technical notes:
1.Dont forget to set the limit switch when you are using it on a cropped sensor camera! .
2. I had to remove the dome shade when using my DP-FE2 with the 5D2. Its OK to leave it on for cropped sensor
3. If you have a Canon 17-40mm zoom ring, it will work fine for this lens in a Subal housing.
4. I used a 18mm extension ring which worked fine. It seems the corners were sharper with this setup than with any other Canon wideangle combinations I have tried. ( just my impression - not formally tested )
The new Sigma lens is incredibly sharp and has less CA than any other macro lens I have used underwater. The autofocus is quite decent - seems at least as good as the Canon 100mm if not a bit better but not as good as the 60mm EFS on cropped sensor. Downsides are the weight - its a brick! My buoyancy collar was a blessing.
Galleries here: http://andresmith.speedalbum.com/Philippin...ovember%202011/ and here: http://andresmith.speedalbum.com/Atlantis%...ovember%202011/