Internal volume
The volume of air inside the mask, between the face and the lens, represents the internal volume. The smaller the volume, the easier it is to balance the mask during your freediving descents
Equalisation
The mask gets pressed against the face when diving down. As you dive, pressure gets applied to the mask's flexible skirt, reducing the internal volume and pressing the mask against the face. This phenomenon, which can cause pain in the eyes, can be avoided by equalising the internal pressure of the mask and the external pressure exerted by the water. Simply breathe out gently through your nose. Air is very valuable in underwatersports, so this equalising effect must be kept to a minimum.
