When mounted on a GDome underwater housing, this 80mm Lens Port Extension is used to extend the distance from your camera lens to the lens element (PDS dome or flat lens). By extending the distance an additional 80mm, the water is pushed further away from the camera lens, allowing the image to effectively be "zoomed out" as if you are using a wide-angle lens without losing any image quality. The additional distance will allow you to make use of the full extent of the PDS dome port. One user has described the experience as "it's almost like having a 10 dome port! It is easy to capture split shots now without having to use the wide setting on my iPhone."
As the Extension Port acts as a massive lens hood over the camera lens, reflections are significantly reduced when using either the PDS Dome or Flat Lens Element.