Thanks nudibranco for trying out the 9-18mm in the 3.5 and 4.33" domes. These have much tighter optical radiuses and so results will never be near as good as with the 4" semi-dome.
Keep meaning to ask, nudibranco, if you could post results with the 8mm FE inside the 3.5" and 4.33" domes, particularly with regard to corner sharpness. I find that below f/8 with the 8mm FE in the 4.33" dome, that the corners get soft quite quickly. I'm wondering how the corner sharpness holds up with the 3.5" compared to the 4.33"??? Easiest to use the corner of the pool and use the tiles or place targets on two walls since the 8mm FE is so wide.