Let's try to clear the waters a bit.
You most likely learned in your open water scuba course that objects underwater appear about 25% larger than they do in air. This is the result of the refraction of light rays as they pass thru the flat plane of your dive mask which forms a barrier between the water and the air trapped inside your mask.
The same rules apply to a camera lens behind the flat port of a housing. Light beams are travelling from water into air, just like your dive mask. For macro photography, this is a good thing. You gain free (as opposed to paying for a more powerful lens) magnification of your subject.
But what about wide angle photography? For wide angle, we're trying to capture a wider area. We want the subject to appear smaller to encompass more area in the image. This is where a dome port comes to the rescue. A dome port, when used under water, creates a "virtual image," which is what the camera records. This virtual image is located close to the camera, usually at a distance of about twice the diameter of the dome from the sensor (or film plane). Using this formula, a six-inch dome will have a virtual image 12 inches from the recording media, and a nine-inch dome at about 18 inches. If your camera/lens combination cannot focus on the virtual image, you're in trouble. Out of focus images are the one thing that can't be fixed in Photoshop.
This is where diopters come into the picture. A diopter when added to a lens, decreases the working distance of that lens, meaning that the lens now can focus on objects closer to the camera. It also generally means that the lens can no longer focus on distant objects, but for this application that is not a concern, as our only concern when using the dome port is focusing on the virtual image.
That's all there is to it. The rule of thumb is flat ports for macro lenses - dome ports for wide angle and general use zoom lenses.
Contact us and we will help you choose the correct port for your needs.