As with everything in photography, it’s all about the light. The dome port is essentially an additional optical element, much like the lens itself, and it will reduce problems such as refraction, radial distortion, and chromatic aberrations.
Refraction is the bending of light waves as they pass through the air inside the camera housing and the water outside the lens port. While light is refracted roughly 25 percent, the focal length of the lens increases by the same amount. You will remember that wearing your facemask under water is different than wearing it on land. Underwater, the laws of refraction cause objects to appear larger and closer - the same distortion occurs with a flat port lens. When a dome port is used, the rays of light pass through the curvature of the dome, avoiding refraction, and allowing the interior lens to retain its angle of view.
Radial Distortion becomes more prevalent with wide angle lenses and large apertures when used with flat ports. The effect is a blur that progresses from the sharpest point in the centre of the frame to the outer edge. The dome port significantly reduces this distortion effect because we have already reduced the problems of refraction.
Chromatic Aberrations are happily avoided when using a dome port, since it is the refraction of light that causes white light to be separated into the different wave lengths of the colour spectrum. Colours within the spectrum do not travel at the same speed, so as light rays travel from water, through glass, to air, they will be unequally bent, causing the different colours to overlap – particularly noticeable on the edges within the image.
Dome Ports Underwater are Different
When using a dome port underwater, the “virtual image” must be focused on, not the “actual subject” itself. The virtual image is created inches in front of the lens with the dome port. This is why the virtual image must be focused on for under/over shots, while using a small aperture (high f-stop) to create enough depth of field to capture the “above the waves” image at the same time.