Water curves against the camera dome, forming a concave shape. Notice that in this side-view the water column has a thick edge and thin center. Water is being shaped like a negative lens.

How do we fix the focus shift problem? Remember that when we combine lenses of equal but opposite power, the result is zero power.

An object may actually be 15 feet away from the dome, but underwater the camera has to focus much closer, usually less than two feet. Photographers use accessory close-up lenses (positive diopters) inside their housings to neutralize the negative power caused by the water curved on the dome.

The cornea of this normal eye allows the person to focus both near and far away. But this naked eye cannot focus as close as someone who's nearsighted.

A person who is nearsighted has a thicker than normal cornea. Just like a camera behind a dome, this naturally nearsighted eye has excess positive power attached. This is why many nearsighted divers can use the MEGA™ series diving masks with their naked eyes.