– Ed
What changes under water?
When light travels from one medium to another medium of different density (refractive index), it changes its path. When light strikes the cornea in air, it bends the light with some help from the lens in the eye, to bring it to a focus on the retina. The cornea is responsible for two thirds of the refractive power of the eye. In water, it becomes a different story because the refractive indices of water (1,333) and the cornea (1,376) are very similar, there is no refraction or bending taking place. The scuba mask becomes very important in this respect. Although it keeps water out of the diver’s nose, it’s primary function is to allow the diver to see, because it places air in front of the cornea. Objects in water are naturally magnified by 33% and this may aid people with mild vision problems.
Colour is selectively and gradually filtered out as a diver descends. This is gradual and different wavelengths will be lost at different depths. There is no abrupt interface. If a diver bleeds at eighteen meters, where there is no red light, it will appear greenish-black. It is interesting to note how underwater photography is affected. Selective filtering by water happens in any direction – not just in the vertical plane. A camera flash will lose most of its effectiveness after as little as 1,5 meters because the light has to hit the object and then travel the same distance back to the camera – 3 meters in all.
What are the optical solutions?
Always alert your buddy if you have to rely on a visual correction.
Contact lenses
A diver cannot wear his everyday eyeglasses underwater for the simple reason that the earpieces of the glasses will not allow the mask skirt to seal on the diver’s face. Both Soft Contact Lenses and Rigid Gas Permeable (RGP) Contact Lenses are popular with divers, because they generally work very well under water. Soft lenses allow the nitrogen absorbed by the eye, while diving to escape. With RGP lenses even more so, because with blinking, the lenses move and mix the tears allowing more nitrogen to escape. The writer has come across reports in the literature that suggests RGP lenses will suction onto the cornea with increased barometric pressure. This notion was dispelled by Dr Dirk Booysen, a Dive Master himself, who informed me that he has fitted many patients who dive successfully with RGP contact lenses. Some precautions with all contact lenses are necessary, such as keeping the eyes closed when removing or flooding the mask. Using Soft lenses, it would be wise to always have lubricating drops at hand and disposable soft lenses would be a good option on a diving trip. It is recommended to flush lenses with saline between dives. Those who wear Scleral Contact Lenses need to consult closely with their optometrists, since tear exchange is limited and there may be issues with nitrogen being trapped under the lenses.
Prescription Masks
Most scuba diving equipment manufacturers offer masks that can be ordered with prescription lenses. Some non-prescription masks may be modified by removing the stock lenses and replacing them with prescription ones or alternatively, laminating prescription lenses on to the inside of the regular diving mask.
A diver who chooses to use a prescription mask must remember to bring his regular eyeglasses to the dive site so that he can see before and after the dive. On extended dive trips, he should consider bringing a second prescription mask as a back-up. In many remote locations, prescription masks are not readily available. Losing a prescription mask can ruin an entire dive vacation.
Over 45 years
For those who require bifocals, or only need help with close-up vision, adhesive magnifying patches, which are applied to a stock mask lens can be used. Alternatively, optical prescription inserts in bifocal form, can be fitted to the mask to provide adequate distance and near vision.
Diving After Eye Surgery
Diving is possible after most types of corrective eye surgery. Before returning to the water after eye surgery, a diver must allow time for her eyes to fully recover. Waiting times vary among surgical procedures, and of course, a diver should attend a follow-up consultation with his ophthalmologist to confirm that his eyes have fully healed before returning to
the water.