The Nauticam 18804 dome port is comprised of the same dome as the 18803 4.33" port; however the design of this port, #18804, elminates the need for an extension ring when seeking optimum performance with the Tokina 10-17mm lens.
The 4.33" diameter dome works well with fisheye lenses, and is a very nice size and light weight, for travel.
This dome port is not compatible with super wide rectilinear lenses like Nikon 10-24 or the Canon 10-22.
This Nauticam dome port is acrylic. We suggest that you also look the Zen DP-100-NT for equivalent performance. The DP-100-NT uses optically coated glass.