part#3 (and last one):
Since I came back from Egypt the Nauticam N85-N120 adapter for Metabones arrived. In addition, I aquired the Canon8-15mm FE (I was dissapointed by the image quality of the Tokina, hopefully the Canon will perform better), a Metabones pure adapter (1x, without optics;MB_EF-M43-BT2) and a Kenko 1.4x teleconverter (KENKO Teleplus MC PRO 300). I did not have a chance to bring them underwater yet, but next Saturday I will go for 10 days to the Adriatic
. Will not be possible to test out everything carefully, but shurely will dry every combination out a little...
Here some observations so far:
#1.: Both Tokina10-17mm and Canon 8-15mm are too big to insert them into the Nauticam housing, when attached to the camera body (similar problem as with the Zuiko 7-14mm). The sequence of operation is: (i) attach the Nauticam adapter to Nauticam housing, (ii) insert camera body w/o lens into Nauticam housing, (iii) attach combination of lens, Metabones (Telecoverter) & Zoomgear to the camera body and finally (iv) attach domeport plus extension.
#2.: The Metabones 1x adapter has different physical length compared to the 0.71x speedbooster (!!; 5,63mm longer). Could not find physical sizes of these adapters on the Metabones homepage to check this before.
=> the Nauticam zoom gears for both Tokina10-17mm and Canon8-15mm do not fit.
=> In the meantime I have designed "Zoom gear adapters" for the original Nauticam zoomgears for these lenses and it works now for both 1x Metabones adapter and also 1x Metabones plus 1.4x Kenko teleconverter (0.71x Metabones plus 1.4x converter does, of course, not make sense) ...
#3.: On surface, both lenses work very well with both metabones adapters and the 1x also with the teleconverter. SIngle autofocus included (I did not test C-AF, since I never use it).
#4.: The angles of view are phantastic, especially with the 1x Metabones and also with the teleconverter (I do, however, not know yet how much the teleconverter will detoriate image quality). For using the FE zoom lenses, Nauticam has obviously made the wrong adapter/housing: a housing for the 1x adapter would have been much better (I ask myself for what purpose this extension was created?).
Here a table that shows calculated diagonal angles of view for different combinations, compared to popular lenses that I have and also others use:

Legend:
RL 7-14: Rectilinear WA,Panasonic 7-14mm
RL 12-40: Rectilinear Zuiko 12-40mm Pro (Arrow indicates that smaller diagonal FOVs as shown in the table are possible).
8-15/1.4x(1.0x/0.71x): Canon 8-15mm with (i) teleconverter&1xMetabones, (ii) 1x Metabones and (iii) 0,71x Metabones (arrow indicates larger FOV's are possible, leading, however, to vignetting).
10-17/1.4x(1.0x/0.71x): similar as above, but for Tokina 10-17mm.
8mm MFT FE: Zuiko 8mm fisheye Pro as reference.
When I have decent UW photos I will post some in a different tread. Can someone please tell me what the optimum format for Wetpixel is?
(=> The photos above are jpg's in original pixel resolution (16 Mpixel), but compressed to max. 1MB. The original ones on my computer show more differences...)
Wolfgang