Dome Port Confusion

I am pretty ignorant of dome port design and theory so to the extent my question reflects ignorance, I apologize.

Nauticam makes a 180mm dome for use with the Olympus 12-40 and 7-14 micro four thirds lenses. To use that dome for the 12-40 lens, Nauticam suggests using a 47mm adapter and a 20mm extension ring. For the 7-14 lens, Nauticam suggests using a 55mm adapter alone.

Zen makes a 170mm dome that accommodates these same lenses, but for the 7-14 one needs to use a 60mm adapter and a 20mm extension, while for the 12-40 the 60mm adapter is used alone.

To put this in a simpler format:

Nauticam dome:

12-40 = 67mm distance to dome

7-14 = 55mm distance to dome

(12mm difference between the two lenses with greater length for 12-40 lens)

Zen dome:

12-40 = 60mm distance to dome

7-14 = 80mm distance to dome

(20mm difference between the lenses with greater length for 7-14 lens)

I realize these are different size domes but I have been told they have the same curvature and are essentially the same glass. The adapters and extension rings are all made by Nauticam and so are interchangeable between the dome port brands

How could both of these domes produce similar results with such different lens to dome distances and how can the two companies have such a significant difference in lens to glass distance between the two zoom lenses?)

Does lens to glass distance simply not matter? Is one of these dome ports "better" because of the differences or is it just a question of physical fit and no optical issues?

Thanks for any help understanding this

I know this may seem a bit confusing but first lets revisit your numbers. If you look at the current Nauticam port chart the 12-40 is listed with a 47mm 85-120 port extension and the Olympus 7-14 is listed with the 55mm 85-120 port extension plus a 10mm extension. That makes both within 2mm of each other at 67mm for the 12-40 and 65mm for the Olympus 7-14. I would expect either configuration to work with both lenses and the 180mm port.

For the ZEN 170mm port as you say the distance for the 12-40 is 60mm and the 7-14mm is 80mm for the port which is 170mm.

This is what you have not factored in, FIRST the Nauticam 180mm port glass sits in a port mount which has about 15mm of extension from the mount to the start of the glass while the ZEN 170mm port mount is about flush with the glass. This difference adds the additional extension for the Nauticam port. SECOND is that the distance from the point of lens to the spot where the virtual image begins is calculated for a zoom lens at its widest angle of view. So 7mm and 12mm. As the 12-40 lens extends to a greater length the dome begins to act more like a flat port as you get close to the glass. Since the 12-40 is about 84mms before being extended and the 7-14 is about 105mm and has internal focus rather that extending is needs to sit further from the port than does the 12-40mm. Because the Nauticam 180mm dome is wider the 12-40 can sit a bit further away from the glass than with the 170mm port. The extensions lengths are about the same but the ZEN port has a flat 120mm opening into the port while the Nauticam port has the same 120mm opening but then angles out preventing vignetteing with the overall longer port. If you look at the two photos you can see the ZEN 200 port on the extensions and the Nauticam 180mm port on extensions with the housing sitting on the tanks.

OK I understand what you are saying. Just so I am sure we are talking about the same thing, the Nauticam port chart I have, dated September of 2016 suggests for the 12-40 that one use either the 55mm adapter and 10mm extension or the 47mm adapter with a 20mm extension and shows the latter as the "optimized" choice. I guess the extra 2mm offers some slight benefit since they consider it optimal.

In the end, it looks like if one wants to use both the 7-14 and 12-40 lenses, the Zen is a better choice because it requires one adapter and extension vs the nauticam requiring either two adapters and an extension ring or one adapter and two different extension rings, one of which appears to require a screwdriver to attach and remove.