Vision is affected greatly underwater. Objects will appear about a third larger and closer underwater. This is because of the way light refracts, or bends, through the mask lens. The magnification of an object depends on the following.
- Turbidity of the water
- Design of the mask lens
- Distance from the mask lens to the diver’s eyes
You need a mask because your eyes are designed for an air environment. Your eyes focus depending on how the light enters the eye. When underwater, the light would enter your eyes differently if you did not have the air gap that the mask provides. As a result, objects would be out of focus. New divers may have a tough time adjusting to the magnification. After a few dives, they may hardly notice its effect.
Look at the image of the pencil in the glass of water. You can see how the pencil appears to be larger in the water.