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UPDATE: There is a contradiction in the instructions for assembling the rig (Canon 8-15mm + Dome port): official port chart states that the dome shade must be removed to avoid vignetting, however the installation instructions for the 5510.11 extension indicate that the dome shade is required to properly secure and lock the port to the housing (tell you that – it’s a one crasy extention!)
So, I talked to Ikelite team and they advise – “… Yes, the port lock uses an extended setscrew that is threaded into the dome shade. Without the dome shade and lock, the port can be accidentally removed from the housing. …. If you have a vacuum pump, you can use the housing with a vacuum of 5-10 inHg. With the housing under a vacuum, you will not need the port lock. With a vacuum of 5-10 inHg it is impossible to pull the port off of the housing. If you do not have a vacuum pump, the port will have to be used with the dome shade so the lock can be installed. The lens will have to be zoomed in slightly to avoid vignetting on the port shade.”
I do have a vacuum pump, so I’m about to risk a $1k fisheye lense and not-so-cheap camera body, all in a noble hope that sucking air out of it is enough to keep the ocean out – wish me luck 🙂