Equalisation during ascent is automatic as excess air inside the mask easily leaks out past the seal.

A good comfortable fit and a reliable seal around the edges of the rubber skirt is important to the correct function of the mask.

National and international standards relating to diving masks provide a means of ensuring that they are manufactured to a suitable quality.

Depending on the depth and clarity of the water, eventually all sunlight is blocked and the diver has to rely on artificial light sources to see underwater.

These may be supported by a relatively rigid plastic or metal frame, or they may be permanently bonded to the rim of the skirt, in a construction known as "frameless".

There is usually a "skirt" of synthetic rubber or silicone elastomer to support the frame and lenses and create a watertight seal with the diver's face.

A nearly transparent skirt provides a greater peripheral vision, though somewhat distorted, and may reduce the feeling of claustrophobia in some divers, but in some cases the light entering through the sides may cause distracting internal reflections.

[9] The section of the mask covering the nose must allow the wearer to block the nostrils while equalising pressure in the middle ear.

[18] Diving masks can be fitted with prescription lenses for divers needing optical correction to improve vision.

[19] Corrective lenses are ground flat on one side and optically cemented to the inside face of the mask lens.

[7][20] A mask is considered to fit well when it seals comfortably and effectively all round the skirt, and provides a good field of vision, without pressing on the nose or forehead while in use underwater.

A low internal volume is considered desirable by freedivers, as less breath is wasted to equalise, and by scuba divers, as there is less tendency to press up under the nose due to buoyancy, which becomes uncomfortable quite quickly.

Too loose may not provide an effective seal and the mask may dislodge easily, and too tight may result in discomfort or pain.

[24] Standard maintenance is to rinse inside and out with clean, fresh water after each day's use, and allow to dry out of direct sunlight before storage.

If the mask is a good fit on the diver's face and the strap is correctly placed, exhaling through the nose will usually drive water out along the bottom edge of the skirt.

If the fit is not perfect, or the strap is too low, contact pressure of the top edge against the forehead may be insufficient to maintain a seal when the head is upright.

Some divers need to pinch the nostrils closed as part of their equalisation technique, and this must be allowed for in the design of the mask skirt.

[21] Early diving masks were made with a single pane of glass or transparent plastic, usually elliptical, but sometimes circular or approximately rectangular or triangular with rounded sides and corners.

It can have the windows closer to the face than the one-window type, and therefore contain less internal volume for the diver to clear or equalise, and a lower buoyancy.

[26] Recent innovations have produced more complex designs, intended to provide extra features: There are other types of equipment for underwater vision enhancement: