Dive Medicine
Presbyopia and Contact Lenses
BY DR. DAVID SAWATZKY
Eyewear underwater. There are pros and cons to consider
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n the last column I reviewed common refractive errors and the effects they have on our vision both in and out of the water. In this column I will continue that discussion by reviewing reading glasses, contact lenses and how they interact with diving.
Presbyopia (far-sightedness associated with age) During normal distant vision the power of the glasses (if required), cornea and relaxed lens combined cause light to be focused on the retina. When you look at a near object, light from the object will diverge at the eye, requiring greater refractive power in order to see the object clearly. This is accomplished by contracting the ciliary muscle to take the strain off the lens in the eye. The natural elasticity of the lens causes it to become ‘rounder’, thereby increasing its refractive power. The closer the object is to the eye, the greater the refractive power required to focus the image on the retina. As we get older, the lens capsule becomes less elastic. For most people, the closest point at which we can focus moves away from the eye about one cm per year (some children can actually bring the tip of their nose into clear focus). Most people will have trouble reading sometime after age 40. The
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nearest point that they can focus becomes so far from their eyes that either their arms become too short to hold the object, or by the time the object is far enough away from the eye to be in focus, the print is too small to read! The only solution is to use a plus lens to increase the refractive power of the system (reading glasses).
Reading Glasses
If the person already needed glasses to see clearly at distance, they will need two pairs of glasses, one for distance and one for close in (reading). These two different lenses are often combined into one pair of glasses with the correction for distance in the top part of the lens and the correction for close in the bottom (bifocals).
In even older people whose lens capsule has become so stiff that the lens has very little ability to change shape, three pairs of glasses may be required: one for
Photo: Trisha Stovel
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