Mary & Don
The reason don is having soft corners is that the dome is not placed at the nodal point of the lens. The dome is designed for the lens and then when you add the tele-converter you move the nodal point out 3/4" or 19mm. If you could put a extension on the dome that would really help the situation. you can read more about the nodal point here: http://www.creativeillusionsphoto.com/virtual.htm
The reason for using the tele-converter is to get the full range of the zoom back. With a 1.4 tele-converter the lens becomes a 14-24 with no vignetting. without the tele-converter you would only have a 14-17 lens without vignetting as seen by the link that Grtz Tobias provided.
I was originally using this set-up with the small Subal DP55B dome and a 20mm EXR, which was when I suffered the soft corners.... Then after discussing the set-up with Ryan at Reef Photo he suggested the 100mm Zen dome and a 28mm EXR.
I had already bought the Zen superdome and had been very impressed with it - so I went for the 100mm and 28mm EXR. I was using it this afternoon on the Liberty wreck in Tulamben, here in Bali and an image is attached below. Still slightly soft in the corners, compared to the Sigma 15 FE and Zen superdome, but not too bad....
The image is a straight JPEG from the NEF - no tweaks at all.
Don
