A few questions about Aquatica locking collar, Canon 16-35, and diopters

Aloha,

Just moved to an Aquatica system from Ikelite (thanks to Meister for the equipment) and have a couple of questions that I'm hoping folks can help me out with.

1) The locking collar. This is something that is not included with new ports, correct? I'm using a 5d3 housing with 9.25" dome and picking up the extensions for the 16-35 and 8-15 in addition to the 100mm macro I have. This is something I would want? Can i move it from extension to extension or will I want one for each?

2) Canon 16-35 extension. Aquaticas lens chart lists the correct one as 18459, yet this does not exist on their website and inexplicably you can't find them by their part numbers at retailers, only their length. Backscatter seems to think it's the 18458, which makes sense as that is the correct one for the 24-70 II and the lenses are the same length. Anyone with firsthand knowledge?

3) +2 diopter. Here's another thing Backscatter recommends with the 16-35 f4, anyone have any feedback one way or the other? What is the purpose of this?

*edit 4) Aquatica has a note that says the 8-15 can't use it's full zoom range with the 9.25" dome. I'm assuming that's at the widest end and would be fine at 15mm yes?

Thanks so much for the expertise. This is my 3rd brand in as many housings, and the learning curve is something I don't particularly enjoy, especially when items are more difficult/costly to get here and need to be correct the first time.

Lyle

I know the +2 is for easier focusing on the virtual image when using the dome. Extensions will be the same if the lenses are the same length at max zoom out.

I think the locking collar works very well, but it would be a pain to switch it back and forth between different extensions

Sorry for chiming in late, just getting back into things here (was out in Indonesia for a few weeks without internet on the boat)

The locking collar is removable, but has Diverdoug1 says, it can be annoying to switch all the time, the macro port does not require one, so there would be the extension for the 16-35mm and the 8-15mm to consider.

The 18459 is a really new extension, just being introduced so it is not common knowledge yet, but it is available.

The +2 can and will help with close focusing on the projected virtual image of any large dome (8" or bigger).

The 9.25" Megadome has a built in dome shade that will show up at the widest range of the lens when used with a full frame camera, but it is fine at 15mm