Started on another promising looking lens today, the new Laowa 12mm F/2.8 FF AF Dreamer a (122 degree) rectilinear. Using the Marelux 230mm optical glass dome with 45mm of extension for this photo at ISO-125, F/14, 1/160th sec. Still working out the best extension length. Any extension suggestions?

The specs say this lens is 76mm long and it appears that the entrance pupil is about 15-20mm below the front element. Using the housing flange to camera flange distance of 39mm and 120mm radius for the 230 mm dome I would say something like a 30-35mm extension.

To judge this the tip of the dome should be about level the dome's aluminium base and that should get you close. If that setup doesn't vignette then shoot a split shot of a straight object sitting on your pool step to see if the width remains the same above and below, like is done in the first sequence in this post:

When the Laowa 12mm is mounted on the Sony A7RC camera in the MX-A7RC housing the front of the lens is parallel to the flange on the housing, I found the best results using a 40mm extension with the Marelux 230mm optical glass fisheye port.

Attached are with the 40mm and 230 port.

3 hours ago, Phil Rudin said:

When the Laowa 12mm is mounted on the Sony A7RC camera in the MX-A7RC housing the front of the lens is parallel to the flange on the housing, I found the best results using a 40mm extension with the Marelux 230mm optical glass fisheye port.

Attached are with the 40mm and 230 port.

That should be about the right spot then assuming the entrance pupil is about 10-20mm below the tip of the front element.