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Hi,
it is actually a Flat Port without any diopter. So it does not change the pictures size. It only reffers to the optical light breaking point from water to air. So optically it will do the same picture distortion as the dome lens, but with a sharp image. All underwater images will be distroted in that way. This is related to the physics of water.
If you would put the flat lens into the dome lens the focus will drop again. The main issue on the blurred image in the hero hd is the fact that the light coming from the water is hitting the dome port first.