By Quentin Bennett
Without correction, vision underwater is worse than on the surface due to the diver’s pupils opening to allow more light into the eye in the lower light conditions prevalent underwater. This can mean a diver can require correction lenses for diving, even if they don’t above water. I personally required vision correction for underwater before I did on land.
When we approach middle age, our ability to focus on near objects deteriorates very quickly (presbyopia). This is the stage when we are most likely to require reading spectacles. The most satisfactory method of correcting this is the use of little reading segments like half glasses, cemented at the bottom of the mask lens. Some divers are happy to have a single lens at the bottom of the mask on the side on which they hang their pressure gauge or consol. This is most commonly on the left side.
For people who also require correction in the distance, we can make up special bifocals. We are unable to provide progressive lenses for underwater use, but are making special corrections that give a similar effect and increase the effective depth-of-field at close distances. This means that we can see little creatures up very close and yet can still see things 50 to 100 centimetres away.
Generally, the best and easiest method of correction are diving mask correction lenses. These are normally ground with a flat front surface that is then bonded to the inside of the mask. Custom-made lens alignment can be set very accurately and several types of bonding mediums are used, including epoxy resin and hi-tech glass adhesives. These types of mask can cover most common issues: astigmatism, long-sightedness (hypermetropia) and short-sightedness (myopia).
Although off-the-shelf prescription masks can be bought, I encourage anyone doing more than basic snorkelling to get custom-made prescription lenses. There are strong arguments against off-the-shelf lenses – mis-centred lenses could give eye-strain, perceptual changes or double vision, particularly in conditions of dull light, low visibility or with nitrogen narcosis. This will be more likely with stronger prescriptions, and could be dangerous.
I studied a large number of random off-the-shelf prescription masks and found the mean lens centration to be far wider than is generally required for the average Kiwi. With only a moderate prescription this error is enough to cause considerable discomfort.
Many prescriptions also include an astigmatic element that cannot be incorporated in an off-the-shelf correction. It can only be provided on a custom-made basis. Prescription lenses don’t necessarily cost much more.
For experienced divers, contact lenses are another excellent method of correction. There are the hydrophilic soft or gel lenses and the hard microcorneal GP lenses that these days are manufactured from oxygen permeable plastics. Soft lenses are probably preferable for most divers because they are less likely to be lost if a mask floods, whereas hard lenses are more likely to be washed out.
Following ascent from a deep dive, hard lenses sometimes have a build-up of microscopic bubbles within the tear film between the lens and the eye. These bubbles outgas slowly through the lens and generally disappear within quarter to half an hour of leaving the water.
Silicone extended-wear contact lenses can be left in for 30 days without removal. This is very convenient for someone going away diving, since the wearer can basically forget them. These days the silicone lenses come in a wide range of spherical parameters, both plus and minus, though they are not available for astigmatic wearers or as bifocals.
Daily disposables are another very convenient type of contact lens for going away diving. They are very safe and trouble-free, and although they appear to be expensive, they require no cleaning or soaking solutions since you just toss them out at night and use a new pair the next day. This both reduces the cost and makes them so much more convenient when travelling, especially overseas. These are the very safest type of contact lenses.
It is very important that wearers do not sleep in lenses other than the correct extended wear because the cornea does not get enough oxygen and is unable to get rid of carbon dioxide. Long term damage can be done. Eyes are far too important to even risk damage and if one is away diving it is doubly stupid to sleep in ordinary contact lenses.
Saturation divers should never wear contact lenses inside a chamber because of the risk of a pseudomonas infection of the cornea. Such an infection can destroy an eye in 48 hours, and this organism has become part of the normal flora of the interior of a chamber.
Refractive surgery is now an option for divers requiring a correction. In the earliest type of refractive surgery, radial keratotomy (RK), a series a radial cuts were made in the cornea. Following RK any diver should be particularly careful about mask equalisation. Mask squeeze could possibly cause stress to the healing cuts which appear to remain fairly weak. RK is no longer commonly carried out.
With photorefractive keratectomy (PRK) and the newer LASIK, the cornea is reshaped using a special laser. Once the eye is fully healed, there is no limitation on normal diving activities.
Plastic, stick-on bifocal lens that are designed to be stuck on the inside of the mask glass are also available. Half-moon shaped and possessing a static charge so that it is molecularly bonded to the glass, it can be fitted by the user. They are produced in the USA in a range of powers from +1.00 to +3.00 Diopters. Obviously anti-static lens cleaners must not be used with these aliphatic polyurethane stick-on lenses.
Do remember to look after your dive mask, whether prescription or not. Broken mask glass is not the sort of thing that number eight wire or sellotape can fix. When on dive trips, store your masks in a safe but accessible place where they cannot be stood on.
Wash your mask well because the mucous and fluids that flow from your nose provide a lovely medium for all sorts of things to grow in. The silicone from which most modern masks are made holds an electro-static charge and actually attracts dirt. Spray-on anti-static lens cleaner will really make a mask come up looking like new.
Remember that your mask is perhaps the most important piece of your diving equipment. It is responsible for your security and safety. Remember to treat both your vision and mask amongst your most precious possessions.