A dome port on any underwater housing for any camera is primarially used to recover the Field Of View that is lost when shooting behind a flat port. A camera behind a flat port experiences a reduced FOV just like you do with a diving mask on. The concentric dome port eliminates this effect. But.. the down side is that a curved port with water on one side, and air on the other produces a convex lens effect. Its called a virtual image. Any camera behind a dome port must be able to focus on this VI to obtain a sharp image. A small dome put on front of a GoPro about the size of 4" or 5" or so produces a virtual image just a few inches out. The GoPro is a fixed focus camera and in low light does not focus close enough to focus on the domes VI. Put a 4-5" dome on a GoPro and underwater object in the closer ranges will be out of focus. Distant objects say 5- 10feet or so may be sharp. It depends on the dome radius. I've done actual tests a small dome on the Hero4 and the image is soft. The larger the radius, like an 8".. the further out the VI and the more chance the GoPro will be able to focus on it.
The deeppro domed housing solves this dome VI problem by putting a close up diopter in front of the GoPro camera. There is a diopter built into the housing just infront of the camera. Basically it puts reading glasses on the camera in order to trick the gorpro to be able focus on the domes VI. The result is a sharp image from close in to infinity. Underwater. Out of water, objects within a few feet are in focus. Distant objects are soft. Like all things in life.. you get what you pay for. This housing shoots like a much more expensive $5000 camera and housing but at less than half the total budget (e.g. lens, camera, housing, dome costs).
Just remember that a dome with no close up diopter will take great in air shots, but the Underwater subjects closer in may be out of focus. It just depends on if the scene is bright enough ambient light for the camera to have a deep DOF and is able to focus on the underwater VI and the radius of the dome.
Now.. if GoPro ever reduces the minimum focus distance of the camera's lens to just a few inches.. then the diopter will no longer be necessary when housed behind a dome port. Many of the people buying the deeppro housing have been tech divers, shedding a larger sized video housing and opting for the smaller package, the 450 ft depth rating and large controls that allow operation with gloves on. By having controls that can actually work with gloves on they are shooting ad-hoc video. Not simply letting the GoPro run the entire dive. The result, there isless video to edit later. And the camera's internal battery runs longer when not recording all the time.
