Thanks Paul and John!
It seems that I have to do my own testing when I get my housing. Today I ordered the Sigma 15mm and plan to compare it with both Zeiss lenses.
But I have some more questions:
1. Is the effect of soft corners (at not fully stopped down appertures) with a dome port as visible over water as under water or is there a difference?
2. Will I get better corner sharpness with the 21mm lens vs. the 15mm? According to the above linked Seacam article less field of view is better for corner sharpness as is more DOF? The question is, which is the dominating effect (for underwater shots, forgetting the special DOF situation for half-half-shots)?
3. Can the 21mm Zeiss lens also be used with a flat port? Assuming there is no vignetting, is there any disadvantage underwater of a flat port besides the ca. 30% more magnifaction and therefore 30% less field of view? For half-half-shots the advantage of the bigger area of the dome port is clear.
4. I would guess that corner sharpness is (with a flat field lens like the Zeiss 21) always better with a simple flat port compared to a dome port, over water and underwater. Is this true?
And one additional comment regarding the 14-24 lens. This lens has a very strong field curvature, at least if focused at close distances. I've posted an example of this lens behavior on Fred Miranda some years :
Lloyd Chambers (digilloyd.com) has also written about the strong field curvature of the 14-24 lens. I've therefore no idea why this lens is so difficult to use with domeports. Maybe the field curvature is just not a spherical curvature as nearly all dome ports have.