The first breath-hold divers probably harvested food from the shallows using goggles made from tortoiseshell polished clear to make a lens. By the 19^th century salvage divers employed bulky copper helmets with big glass windows, called lights, to work by. In the 1930s the sport of goggling had arrived in France and would-be explorers were adapting welding and flying goggles to explore the Mediterranean. The modern diving and snorkelling mask serves exactly the same purpose as that first designer tortoiseshell eyewear of millennia gone by. It removes the water through which our eyes can't focus and replaces it with an air space through which they can. Without that air/water interface, the underwater world would be unfocused. Dive masks also enclose our noses providing three important benefits. Preventing water entering our nostrils and sinuses is one. Another is that any water that gets past the seal can easily be expelled by simply exhaling through your nose. Finally pressure building up inside our bodies and outside the mask can be balanced by breathing out into the mask's airspace.

Given that our primary sense underwater is usually vision, and we go to so much effort to dive in order to observe the undersea world for no other reason than our pleasure, it's surprising how little thought we sometimes give to selecting a face mask.

A diver needs to be able to see well underwater. With a mask on, your vision is always going to be compromised to some extent. Understanding how it could be impaired will help you select a key feature of any face mask wisely.

Clean forward vision is important. It's useful to check your mask doesn't create a tunnel effect. Modern masks usually provide excellent vision to the front. It's your downward and peripheral vision that is more likely to be restricted.

For example the lower edge of the mask skirt can interfere with looking down. There's a lot of equipment usually mounted between your chest and waist. This includes the releases for your BCD and weights, and harness-mounted accessories like instruments, delayed SMBs and reels, knives, torches and cameras. Being able to clearly see this area makes it much easier to operate your releases, read your gauges and remove and replace equipment.

Peripheral vision is also important. Good peripheral vision is vital for checking on your diving partner and maintaining awareness of possible entanglement or other hazards when finning through narrow gaps like pier supports, coral grottos and shipwreck companionways. You also tend to "see" movement through the corners of your eye, drawing your attention to fish for instance. Normally when you are swimming underwater you are horizontal, meaning your head is tilted backwards as if looking up on land. This restricts your head's sideways movement a little, making optimum side vision through your facemask even more useful.

Masks are made up of three main features. The faceplate, skirt and strap. A mask will also have a way for you to block off your nostrils so that you can clear your ears. They may also be equipped with purge valves and perhaps have lenses installed to correct for eyesight problems. Here's a rundown on these features in a little more detail. The faceplate will almost always be made from tempered glass. If you are unlucky enough to break the glass it fractures into large relatively blunt pieces. This is a safety benefit. Ordinary glass would shatter into small sharp slivers and would clearly be very dangerous. Plastic is sometimes used, mostly in children's masks. Plastic is easy to scratch and less easy to keep from fogging up than glass. But it is harder to break making it more forgiving of rough handling.

Spearfishermen sometimes use specially darkened windows to shield their eyes from their quarry. Many animals, including fish, seem tuned into eye contact and find it threatening. This kind of mask might also find favour amongst naturalists and underwater photographers to help them interact with subjects more easily. A downside is that it can make LCDs difficult to read, so it's worth testing with dive instruments and camera readouts first.

Before first use, wipe toothpaste on the inside of the glass. Releasing agent used in the manufacture of your mask can leave a residue on the faceplate. This makes it hard to stop the mask misting up. You only need to do this once. Toothpaste is mildly abrasive and removes the residue. Then rinse your mask out thoroughly. Applying a liquid defogger before each dive will then prevent any misting from condensation.

The mask skirt is in direct contact with your face. So it needs to be comfortable. If it isn't you'll really start to feel it during long and repetitive dives. The skirt is also responsible for making the seal. A mask that does not seal well is not just irritating - it can be dangerous (a diver was killed after failing to seal his mask under his wetsuit hood properly and running out of air as he continuously tried to clear it). At the very least it is distracting taking your attention away from diving and spoiling your enjoyment.

The more supple the skirt is, the more comfortable it will be and the better it will seal. You can easily roll the edge of the skirt between your finger and thumb to compare models in the store. Cheap masks often feel quite rigid when you try this. Better ones have much more give.

Modern mask skirts are normally made from silicone. Introduced to diving a quarter century ago, silicone is hypoallergenic, making it better suited to sensitive skins than neoprene. It is also much longer lasting. Original silicone diving masks are still in use. Today neoprene rubber is only used to make the cheapest masks.

Clear skirts are popular. They let extra light in through the sides and sometimes let you see shapes off to the edges. Some divers find these less claustrophobic. Black skirts are favoured by underwater photographers because stray light diminishes their view of the camera viewfinder. Spearfishermen also like them. They often search under overhangs and in holes and don't want bright sunlight from the periphery preventing their eyes from adjusting to the gloom.

Even a shallow dive to a depth of 3 metres increases the pressure around your body by nearly 5 psi. Most of your body is comprised of liquid and this increase goes unnoticed. But your middle ear is air-filled and separated from the water pressure by your eardrum. As pressure mounts around your middle ear it hurts! Fortunately it's easy to avoid. A gentle exhalation will send air into your middle ear through a tube in your throat to keep pressure inside and outside your middle ear balanced.

Many divers find it easier to clear their ears if they can block off their nostrils to create a back pressure. Masks usually have either a nose pocket or finger wells that make it easy to reach your nostrils. Though many divers squeeze their nose closed between finger and thumb, pinching can become uncomfortable, especially if you are breath-hold diving and making a lot of descents. It can be easier on your nose if you just block your nostrils from underneath.

Some masks have purge valves. These simple mushroom valves let you clear water through the mask base very easily. They only work when the valve is at the lowest point. You cannot clear when upside down or on your side. Whether you can clear your mask without pressing it against your face (hands free) depends on the mask and how tightly it is strapped on.

A few non-purge masks will also clear easily without you having to hold them against your face. This is a nice feature. Most masks have to be pressed firmly against your face in order to clear them. Otherwise exhaled air just seeps out through the upper skirt without forcing the water out. If your mask is leaking and your hands are occupied deploying a reel or assisting another diver, then you may have to improvise to press your mask against your face in order to clear it.

The mask strap can be worn under a hood. This makes it much harder for your buddy to kick it off your face. However making it difficult to remove can also interfere with making a rescue or being rescued if resuscitation is involved. Straps are split to cup the head which increases comfort and makes them fit more securely. Some divers favour slap straps, which are less likely to get tangled in long hair.

Most masks have clips that make it quick and easy to adjust the strap with one hand - even underwater. This can be a useful feature as you might want to slightly over tighten your mask strap for entering the water and descending through current, then slacken the strap for more comfort during the dive itself.

Full-face masks use a spider - typically five straps to ensure the mask stays on. If a diver loses consciousness, then the mask should remain in place. For as long as the diver has gas, the casualty can continue to breathe.

There's a bit of a myth here. Low volume is often touted as an important selling point for masks. For a scuba diver mask volume is not normally a concern. There's plenty of exhaled (waste) air to equalize the mask during descent and for clearing any water that leaks in. So mask volume is not nearly as much an issue when choosing a mask as overall vision and comfort. Even the largest wraparound masks can be totally flooded and cleared four to six times on a single breath.

For breath-hold divers mask volume is crucially important. With only the air in their lungs available to equalize the mask and many freedivers regularly diving to 30 metres or more, minimum air space is vital. Freediving masks are ultra compact. Even so, some divers pack out the mask with putty to fill in the remaining internal air space.

Because freediving masks have their lenses set so close to the eyes, they are popular with underwater photographers who use SLR cameras or video cameras as they make it much easier to see into the corners of the viewfinders.

Extreme freedivers use special contact lenses. These eliminate air spaces altogether and therefore don't require valuable air for equalization.

Snorkelers and scuba divers can choose from a wide range of face masks, making it easy to find one that best meets their personal requirements. Most are fine for general recreational diving, others may offer specific benefits for specialist activities like freediving and underwater photography. Snorkelers and recreational divers usually use half masks. These only cover the eyes and nose. Military, professional and advanced sport divers may use full face masks. These also cover the mouth. Full face masks can provide protection for working in contaminated waters. They also make it easier to speak clearly when using microphones. The large airspace inside a full face mask can create a build up of carbon dioxide, so full face masks use oro-nasal pockets or a mouthpiece and nose clip to ensure exhaled gas isn't rebreathed. The classic mask. Monolens masks have a single faceplate. Large oval masks have largely been dispensed with because of their bulk and poor downwards and side vision. They are sometimes still chosen by underwater photographers to dress their models because of the clear view they give of the diver's expression.

Contemporary monolens masks are much smaller and offer much better vision than their predecessors.

Wraparounds have side windows intended to increase peripheral vision. They are usually larger than single lens and two lens designs. Some wraparounds have lenses set at 45 degrees. The sloping lens design lets you see what is immediately beside you. Others have the lenses set at 90 degrees. These prismatic versions actually let you see things slightly behind you. Prismatics are excellent for dive leaders as you can see divers following behind with just a slight movement of your head.

They are not recommended for underwater photography with SLR cameras. The distance from your eye to the faceplate can make it hard to get your camera's eyepiece close enough to see the viewfinder clearly. Light streaming in through the side windows can also impair your view of your subject and camera displays. With consumer digital cameras where you look at a large LCD monitor these larger masks are fine. Videographers will find these masks are generally well suited to housings with monitors - with small viewfinders they will be difficult to use.

Split lens designs are usually the most compact types of face mask. With these designs each lens is inset separately in the frame. Monolens masks have to be larger to let the faceplate span the bridge of your nose. Split lens masks don't have this restriction and can shave a little extra off the internal volume. They have the advantage that they are also easy to fit with off-the-rack corrective lenses. Once you've narrowed down your choice of masks, you'll need to make some checks to test them for their field of view and to see if they seal well. A few tips will help you out if you are new to the sport. To test a mask for its vision qualities, it's worth putting on your BCD and checking to see how good your downward view is really going to be. Vision to the sides can be checked by stretching out your arms horizontally and raising your thumbs. Swing your arms backwards to the point where you lose sight of your thumbs. These checks give you a benchmark for comparing different masks. To test your mask for a good seal there are a couple of steps you can take. Sweep back any hair that could get trapped under the skirt. Place the mask gently against your face. You should feel the skirt making contact evenly against your face all around. Inhale very gently and the mask should seal. For as long as you inhale it should stay on your face without you having to hold it. If you have to suck hard to keep the mask on, you'll probably have to over tighten it in the water. This will make it uncomfortable and it will probably leak anyway. You should be able to wear your mask loose. The skirt is designed to work without you having to over tighten the strap. Over tightening straps tends to force the skirt against your face and this causes the skirt to deform and leak.

This is a good basic test. Many dive shops and instructors recommend it. To make it more reliable it is also worth trying this test using a regulator or snorkel mouthpiece. This is because the mouthpiece sits under your upper lip and can break the seal leading to leaks.

Facial hair can be a problem. A supple mask skirt will seal more efficiently than a more rigid one as it can mould better across the fuzz. If you cannot find a mask that seals well over your moustache you can use gels to solve the problem. If your vision isn't perfect, it's easy to correct. You can use contact lenses provided these won't trap gas. If they do then nitrogen bubbles can form in your eyes and fog your vision during the ascent and after the dive. Your optician can advise you on which contact lenses are safe to use while diving.

Many masks can be fitted with off the rack lenses for basic eyesight problems. At Mavericks we prefer to have our clients' exact prescription made up and bonded to the mask glass. We feel this is better than an off-the-rack solution (we don't buy our own eyewear that way). It also makes it easier to deal with astigmatism, long sight and bifocal requirements. Bonded lenses can be fitted to almost any mask, so you are less likely to have to compromise on the mask you finally choose.

We recommend you consider buying a back-up mask if you need vision correction - just as you would own spare glasses or contact lenses.

Although being underwater does magnify by about 25%, you still need good surface vision to find your buddy if you become separated, evaluate exit conditions and see boats.

We recommend you try your mask in a pool as a final check. This is especially important before you commit to having lenses installed. Most dive shops, including Mavericks, have an easy going policy on testing masks. If it doesn't fit properly you can simply exchange it. Because vision is so important and losing your mask could make it impossible to deploy a marker buoy or read your gauges to make a decompression stop, some divers carry a spare mask. If they lose their primary mask they can fit and clear their back up in seconds. Masks need little maintenance. A fresh water soak is usually enough to keep them fresh. It's wise to carry a spare strap - these break from time to time and you don't want to miss a dive (and probably force your buddy to miss out as well) for the sake of such a cheap item. A protective box is often included when you purchase a face mask. If not, it's a good investment to buy one. Your mask box is also a good place to store your spare strap and defog. Try to keep your mask out of the sand on the beach. Grains can get in between the frame and the skirt and cause leakage.

Masks are very cheap. Yet their application is out of all proportion to their cost. Time spent selecting your mask is always well spent. After all it's been said they are your window into the sea.