By Paul Clarke, Vision West, Karrinyup, Western Australia
SCUBA diving provides an opportunity for us land-locked-beings to become slippery Neptunian creatures in a visually tantalising alien-like theater of movement and colour. Being able to view the underwater world has to be the main attraction for taking up the sport. Not being able to see what’s under the surface would therefore make the experience not much more than a cold sensation accompanied by a dull bubbly sound.
So considering SCUBA diving is a visual experience, which eye conditions are likely spoil the adventure and what visual aid options are available?
There are four main eye conditions that may need to be corrected for diving - shortsightedness, long-sightedness, astigmatism (eye shape), and presbyopia (age related up close focusing).
Mild amounts of shortsightedness and presbyopia seem more tolerable under the water than on the surface and even moderate degrees of long-sightedness and astigmatism appear less of a problem for most SCUBA divers. This is because; water, being denser than air (therefore having a higher refractive index) produces a slight magnification.
For those divers that do require visual aids when diving, the options are – Purchasing a pre-made prescription mask, have your prescription inserted into your mask by an optician, or wear contact lenses. Obviously wearing regular spectacles while diving is not an option, notwithstanding a product called a “Diving Spectacle” that looks like a pair of glasses without arms and a sucker attached to the front for mounting inside a full faced mask. The bottom line on this product is it is ineffective and looks quite silly.
Pre-Made Prescription Lenses.
Most mask manufacturers supply a range of pre-made prescription lenses to fit their masks. Depending on the brand, the powers available begin at –1.50 dioptres and go up to –6.00 dioptres and in increments of 0.50. There is also a limited supply of powers up to –8.00 dioptres (“dioptres” are the unit measurement for lens curve and power). The pre-made lenses correct shortsightedness only, although there are some pre-made bifocal lenses available for correcting presbyopia.
Although pre-made lenses are convenient and usually more cost effective than lenses from the optician, the pre-made lenses are not likely to provide an exact correction for the wearer. This is however not necessarily a problem; wearing an accurate prescription in a diving environment may not deliver significantly better clarity for the diver. The small amount of time a diving mask is normally worn also lessens the likelihood of being bothered by a slightly inaccurate prescription.
The best way of finding the most appropriate pre-made prescription for your mask is by asking your optometrist for your “best sphere” prescription for each eye. Let your optometrist know that the pre-made lenses are available in 0.50 increments.
When are pre-made lenses not suitable?
If you are shortsighted and have astigmatism greater than 1.00 dioptre you may find your clarity of vision using pre-made lenses inadequate. Double vision may also occur if the distance between your pupils are considerably different to the generically set distance of the optical centers of the pre-made lenses; this is more likely to manifest in higher powers.
There are no pre-made lenses available if you are long-sighted, or significantly astigmatic.
Because pre-made lenses are made in single vision form only, shortsighted people over the age of forty-five are likely to have problems focusing up close when wearing them.
Which lenses can be made-up by an optician and what will they look like?
Concave, or minus lenses, are used to correct shortsightedness. These lenses have a steeper inward curve on the back of the lens and lesser outward curve on the front. Concave lenses used in diving masks actually have a flat front surface that is bonded to the back of the existing mask lenses; pre-made lenses are assembled this way also.
Convex or plus lenses are used to correct long-sightedness and presbyopia. The front outward curve on these lenses is higher than the back inward curve. In spectacles, plus lenses have the convex surface facing away from the eyes, on a diving mask however, the lenses are glued on the inside, therefore the convex surface faces towards the eyes.
Lenses that correct presbyopia are also convex. They are cut into a semicircular shape and mounted in the mask at a height that allows the wearer to easily view the dive computer but low enough so they don’t interrupt the distance vision.
Shortsighted and longsighted people that are also presbyopic will need bifocal lenses mounted in their mask in order to focus up-close. Modern progressive power lenses can’t be used in diving masks as the greater than usual distance between the eye and the lens surface is not compatible with the intricate function of these lenses.
All prescription lenses used in diving masks are made from glass and bonded on the back surface of the original mask lens using a UV curing adhesive (Loctite 358). The reason the lenses are mounted on the inside is to keep the lens curves in air. If they were mounted on the outside of the mask the water would neutralise the effect of the lens; the same reason our vision becomes blurry under water when not wearing a mask.
Can prescription lenses be fitted to any kind of mask?
For cosmetic reasons split-lens masks are more appropriate for converting to prescription, however lenses perform just as well in a full-face mask. There probably isn’t a mask around that can’t have prescription lenses fitted to it.
Contact lenses provide a good alternative to a prescription diving mask. It also allows the wearer to maintain clear vision once the mask is taken off.
Although all kinds of contact lenses are worn by SCUBA divers, the recommended lenses to use are “daily disposables”. Micro-organisms that exist in seawater may remain in contact lenses after a dive. This could cause an eye infection, which even has the potential to be serious. Also, when a contact lens is lost during a dive, as they sometimes are, the replacement cost of daily disposables is far less than other contact lens designs.
Can I claim the mask lenses through my health insurance?
Yes, prescription lenses are prescription lenses no matter what type of frame they are fitted to and can be claimed on your health insurance when purchased at a certified optical provider.
If you need any advice regarding the best option for your visual needs while diving, please feel free to contact Paul at Vision West on 9445 1499.
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