June/July, 2008 Your divemask 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.
Many divers require correction lenses for diving. In the lower light conditions prevalent under water the diver’s pupils open to allow more light into the eye. Underwater, therefore, uncorrected vision is worse than in
the brighter conditions above the surface.This usually means that a diver’s vision underwater without correction is worse than on the surface similarly uncorrected.
Diving mask correction lenses are normally ground with a flat front surface that is then bonded to the inside of the mask. This is generally the best and easiest method of correction. 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. Astigmatic correction can be included as well as plus corrections for long-sighted
(hypermetropic) divers.
For scuba diving there are strong arguments against off-the-shelf lenses, firstly because there is no individual control of the position of the correction lenses. Miscentred 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. The New Zealand Standards for
any lens over 2.50 dioptres require that it be centered correctly within 1 millimetre.
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 and I encourage anyone doing more than basic snorkelling to obtain custom made prescription lenses. These don’t necessarily cost much more.
Secondly most corrections include an astigmatic element which cannot be incorporated in an off-the-shelf correction. It can only be provided on a custom made basis.
For experienced divers contact lenses are another excellent method of correction. There are the hydrophyllic or silicone soft lenses and the less common hard microcorneal GP (gas permeable) lenses that are
manufactured from oxygen permeable plastics. These days most soft lenses are disposables.
Very generally, soft lenses are preferable for diving because they are less likely to be lost in the event of a mask flooding. Hard lenses are more likely to be washed out in a flooded mask, although experienced
divers may use them when diving, provided they are fully aware of the risk of loss.
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 material and they
generally disappear within quarter to half an hour of the diver leaving the water.
Some silicone extended wear contact lenses can be left in for several days without removal. This is very convenient for someone going away diving. The wearer can basically forget them, in practise, therefore being
normally sighted whilst wearing them. These days the silicone lenses come in a wide range of parameters, both plus and minus.Daily disposables are another very convenient type of contact lens for going away
diving. At the end of the day you simply take out your lenses and toss them away. Next day you insert a new pair. They are very safe and trouble-free, and although they appear to be expensive, they require no
cleaning or soaking solutions. This both reduces the cost and makes them so much more convenient aboard a liveaboard or in a motel or the like, 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 types. With the eyelids closed the cornea does not get enough oxygen and is unable to get rid of carbon dioxide. Long
term damage could 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 the 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 had to 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 LASIX the cornea is reshaped using a special laser. The cornea heals very quickly, and once fully healed there is no limitation on normal diving activates.
When we approach middle age our ability to focus on near objects deteriorates very quickly (presbyopia). This is the stage when we require reading spectacles. In dim light, such as underwater, the problem of sharp
focus on near objects is greater because our pupils open to let more light into the eyes. The depth of vision is reduced even more than on land, and we need lenses to read gauges and set cameras. I personally
required a near vision correction for underwater before I did on land. 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. There is now a plastic stick-on bifocal
lens that is designed to be stuck on the inside of the mask glass. Half moon shaped and possessing a static charge so that it is molecularly held on to the glass, it can be fitted by the diver himself. 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. The optical characteristics of this type
of lens is not as good as glass, but it sufices for many divers.
For people who also require correction in the distance special bifocals can be made up. 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 cm away.
Many modern masks have easily removable glass and this is essential for the bonding of correction lenses. Minus lenses are easily ground in New Zealand but plus lenses for long sighted divers have to be ground
overseas, with a consequent delay.
Do remember to look after your dive mask, whether prescription or not. A broken mask glass is not the sort of thing that No-8 wire or cellotape can fix. Whenever I organise a trip I take along a plastic storage box or
crate large enough to take all the divers’ masks and computers. This is kept in a safe but accessible place where it cannot be stood on.
Wash your mask well because the mucous and fluids that flow from your nose provide a perfect 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 spectacle lens cleaner will really make a mask come up looking like new.
Remember that your divemask 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.
The author of this article, Quentin Bennett FCOptom, DipCLP, FAAO, MRSNZ, is a Napier optometrist who has worked on problems related to vision underwater for some years. Military work in the UK and the USA
included studying the eye’s shape under increased hyperbaric pressure and designing and fitting specialised contact lenses for military purposes. These replace a facemask, allowing a diver to see equally in air o
r underwater. He is also a much published underwater photographer
By Quentin Bennett
"Corrective Lenses"