QUOTE (ScubaLute @ Jan 25 2010, 01:02 PM)

The Nauticam website links to a PDF (look on the extension rings page) that demonstrates how the extension rings attach to dome ports. There is a locking mechanism in the extension to connect it to the port securely. The other observation/question I would have is that it looks to me that extension rings cannot be shared between macro and dome parts.
There are two different types of extension rings available from Nauticam... here's the differences and uses:
- Locking Extensions:
These show up in the Nauticam nomenclature as, for example, "Extension Ring xx", where the xx indicates the length in millimeters. The locking port extensions work with all of the Nauticam domes, plus the dedicated Macro 60 port and Macro 105 port... plus they work with the port adapters, so you could use them with your Subal or Ikelite, etc port. They use a locking bayonet, the same bayonet that the housing port attachment uses. The extension bayonets have a small locking lever. Since these rings can be used with both macro and dome ports, I can see them getting used a lot.
- Threaded Extensions:
These show up in the Nauticam nomenclature as, for example "Compact Port Extension xx", where the xx again indicates length in millimeters. These extensions work in conjunction the Standard Port Base plus a Compact Port. This is a flexible and lightweight solution, but does not work with the dome ports nor the Macro 60 or Macro 105 ports. The compact port and extensions are threaded. The Standard Port Base is threaded on the outboard side, and has the bayonet on the inboard side as above. Using different sized extensions, you can cover a lot lenses, but since these only support the flat port, you won't get the wider lenses with this setup. The main advantage here is the lighter weight. The extension rings are less money than the locking version above, but I think when you factor in the cost of the two piece port it is pretty much a wash.
Unless the weight is a primary consideration or you have a lot of different lenses that need a flat port, I'm leaning towards recommending the first approach. Hope that helps...
Chris