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Hey @Lena
Not sure you’ll find many other Ikelite users having experimented with other port extensions than those recommended by the manufacturer’s port chart.
Underwater lens performance is definitely affected by not having the optimal extension, and I’d trust that Ikelite did adequate testing prior to recommending the #5510.11 extension, to be used with your 8 inch dome.
Clearly, the #5510.22 extension is too long. I now understand why you have those black corners in your fisheye photos: the lens sits too far back, compared to the dome port. Not surprising that it helps the lens focus, since the virtual image will be further away than it is with the correct extension.
My guess is that you haven’t been getting satisfactory sharpness with the #5510.11 extension, not because it’s unsuitable, but because of the camera struggling to auto-focus.
I suggest you try using your fisheye with the recommended extension again, focus manually (pre-focusing on your fin, or anything that will be about the subject distance), and then see if you’re happy with the sharpness (make sure to shoot at F/13 minimum). You can open up when there is no important features/details in corners.
Regarding your 16-35mm lens, I can’t see anywhere Ikelite describing the actual extension length, for the 2 extensions (5510.22 and 5510.24), they only specify a recommended length of the housed lens. To be 100% certain, worth reaching out to them to confirm whether they expect any adverse performance, or simply do your own testing (same as above, manual focus to rule out AF errors).
Keeps us posted!