Scuba Diving International (SDI) AFS 270 Practice Test

The mask’s main job is to create an air space in front of the eyes so you can focus underwater. Eyes focus best when light travels through air; when you’re submerged, the surrounding water alters how light enters the eye, making it difficult to focus and causing blur. By trapping air between the eyes and the water, the mask restores the familiar air–eye refractive conditions, so objects come into clear focus again.

Keep in mind the mask also seals to keep water out and often includes a nose pocket for equalizing, but the primary function is that air space for proper focus. Some minor magnification can occur because of the air pocket acting like a tiny lens, but that isn’t the main purpose.