Optimal Dome Port Extension – Canon RF 15-35

I've recently sold my compact camera setup and decided to take my R5 underwater. There are some new considerations for me here – and I would appreciate some peer review. I've just watched Dr. Mustard's excellent presentation: 'Beyond the Dome – Dome Port Theory' to wrap my mind around the basics, and read other threads here on WP re: these topics (dome position, pupil location, etc.) but wanted to pull it all together here.

I want to be sure – to the best of my knowledge – that I've positioned the RF 15-35 correctly in the Aquatica 230mm megadome. Manufacturer guidance is to use a 70mm port extension. The lens sticks ~30mm into the dome (at 15mm zoom) with this extension.

To check this was positioned correctly I needed to know the entrance pupil location. I measured this using a standard 'point of no parallax' test often used by panorama photographers. The measurement agrees with where I can see the pupil, sitting ~113mm from the base of the lens (right at the 'L series' red line) or ~30mm from the front element. 

Set in the housing with the 70mm port extension, the pupil is parallel to / sits at the dome opening. As I understand: the entrance pupil should be positioned at the dome's centre of curvature. The dome is not a full half: the centre of curvature is ~3.5cm back from the dome opening (ironic – considering the attached figure used in Dr. M's presentation – the 'wrong extension' here was 3.5cm off). 

Either I've missed something, or I need another extension to move the lens back. 

Related (but I don't want this to hijack the thread): not interested in the WACP (that's taking cost a little too far). I will likely purchase a fisheye lens as well – but would prefer a native RF mount, so might be waiting for a few years (and I'll still need to position it correctly... )

It can be a balancing act setting up domes correctly - as I understand manufacturers will test the lens in the dome at its widest zoom and find the best extension that does not vignette.  The 170/180mm domes for example  typically need to place the entrance pupil in front of the radius of curvature to prevent vignetting as they are not full hemispheres.

If you want to check the position you need to know the radius of curvature - for an non-hemisphere it's probably not 230mm/2, I have seen it stated that all the 230mm domes have the same radius of curvature - 120mm from memory, but you can calculate it if you have the dome in your hands from circular segment formulas. 

You can test for vignetting in air if you can rig up a stand for the camera which allows you to move it back.

Where did you get your data for extension length for the 15-35, aquatica in port charts only specify the part number 48472, which seems to be specific to the 15-35.  It's not listed on the extension ring page of their website.

A quick perusal comparing extensions specified for EF lenses and the 230mm domes from Nauticam shows the amount specified is very close and Nauticam specify 70mm for the 15-35 lens. 

Thanks Chris. I have the dome and extension they sent me -- so my data comes from direct measurements. 

The extension measures 70mm – same length recommended by Nauticam for the RF 15-35. But I also see that Nauticam recommend the same extension for both their 180 and 230mm domes (guess they're rounding to the nearest existing extension).

For the centre of curvature I made an assumption that the diameter of the dome port (230mm) is close to what the diameter of the full sphere would be (this will be an underestimate, but close to true). Laying my dome flat to measure the height (85mm) and comparing this to the radius (115mm) I can see that the centre of curvature should be ~30mm back from the opening (35mm if we use the commonly stated 120mm radius). 

I can move the lens back 30mm and it does not vignette. The front element in this position sits at the dome opening and the lens only has a 110 degree angle of view. 

I would assume that the manufacturer arrives at the correct extension length using similar calculations and hopefully some testing. But that means I'm wrong – and I don't see any errors in my reasoning.

Edit: I've read some incredibly detailed posts by folks here on WP re: lens position in dome ports. Cunningham's law ('the best way to get an answer on the internet is to post the wrong answer to the question') suggests I've not made any mistakes here...