Nauticam optical glass dome ports are finely grounded to the highest standard of precision and represents the state of the art manufactering and optical technology.  It features in total 4 sizes to cater different lens.  And 230mm optical glass wide angle port is particularly good for both DSLR ultra wide angle lens and fisheye lens for example:

Nikon AF-S NIKKOR 16-35mm f/4G ED VR

Nikon AF Fisheye 16mm f/2.8D

Nikon AF DX Fisheye 10.5mm f/2.8G ED

Sigma 15mm F2.8 EX DG Diagonal Fisheye

Canon EF 16-35mm f/2.8L II USM

Canon EF 16-35mm F4L IS USM

Sigma 15mm F2.8 EX DG Diagonal Fisheye

Canon EF-S 10-18mm f/4.5-5.6 IS STM

Specifications:

Body construction: anodized aluminium alloy

Port shade: BS/Aluminium alloy

Coating:  multi layer braod band anti-reflective coating inside. 

Depth rating: 40m

Weight: 1.63kg

Port mount: N120 

Radius of Curvature: 120mm