back after a couple of dives...
it seems, that i can't get sharp edges with the lens and the "small" 170mm-Port on the D800.
Tokina +20mm Extension + Athena
Tokina +40mm Extension+ Athena
Tokina +Kenko 1.4 +20mm Extension+Athena
Tokina + Kenko 1.4 + 40mm Extension+ Athena
all settings corners too soft... worst at 10mm, slightly better at 17mm. Can't fix it, stepping down to f11/13 doesn't make much difference.
I think i need a new Port...
Any suggestions? It's a Sea&Sea MDX-D800 housing. I'm looking at a S&S FE 240mm Port and the Nauticam 8.5" Dome at the present. Will the Nauticam be adaptable for the S&S-housing? Which one would you buy? (and no: ZEN is out of range...)