What limits it to approximately 100 degrees give or take is the flat interface, several of these type lenses in the past, depending upon what camera they are used on etc were as wide as 110 degrees, I think the Zen wide angle used on some Patima housings falls in that catagory as well, a large, nice piece of equipment. None of these lenses are actually rectilinear lenses, in fact, when used above water, if you notice they behave much as a fisheye lens and you will note also the curved lines. Once underwater, regardless, they bow to the laws of optical physics, to get a DFOV much beyond 100 degrees without severe distortion a dome is required.
Without knowing your shooting distance etc it is difficult to calculate anything accurate, I would say it looks like a 100 degree lens and of course you can do some geometry with a known shooting distance and a scale for a subject and calculate it pretty darn close.
I shot a lot of test photos with various lenses last year doing exactly that from a known and consistent distance using a ruler in the photo, many, many more than I posted or would post since they are hugely boring. But it is the only way to really find out what the true working FOV actually is with a camera and a housing.
I came real close to buying one of these (XZ1) but I think I am over it now, I think.
N
the link, tnks 
