John F. Kent, O. D., F.A.A.O.
"If you want to keep flying, you must wear glasses." These are dreaded words for most pilots. Some of this fear of wearing glasses may be from past experiences of either witnessing or hearing about some pilot making a "spectacle" of himself as he tried to fly while adapting to his new glasses.
The eye, just like the rest of the body, does not remain stable throughout life. As the body changes, so to does the eye change. There are several ways the eye changes during our lifetime. One is the refractive error or focusing of the eye at distance. Another consistent change is the decreasing ability of the eye to focus at near as we get older. Almost everybody in their forties will need some type of reading prescription.
There are four common types of refractive errors. Nearsightedness, or myopia, is a condition in which the vision is more blurry at distance than near. In myopia, light is focused in front of the eye's retina. The retina is the layer of light receptor cells which line the inside of the eyeball. In myopia, a minus (concave) spectacle lens is used to properly focus light on the retina. Farsightedness, or hyperopia, is a condition in which the vision is more blurry at near than at distance. In hyperopia, light is focused behind the eye's retina. In hyperopia, a plus (convex) spectacle lens is used to focus light on the retina. A farsighted individual may be able to focus (accomodate) an extra amount and obtain clear vision at distance and possibly even at near. This extra focusing frequently leads to eyestrain. Astigmatism is a condition in which there is unequal focusing of light inside the eye. This unequal focusing may cause blurriness at distance and near. A toric spectacle lens is used to focus light on the retina in astigmatism. Presbyopia is a condition in which the eye is unable to focus properly at near due to aging changes of the eye. This condition occurs in virtually all individuals over the age of forty. Reading glasses or bifocals are commonly prescribed for this condition.
It is very common for several of the above conditions to occur at the same time. For example, an individual may be a myopic astigmat and a presbyope. This person would probably wear bifocals that would improve their vision at distance and near.
Corrective lenses compensate for the natural refractive error of the eye, but while doing so, they may also cause slight changes in a person's visual perception. Unless the prescription is unusually high, most people can readily adapt to these distortions that may make the visual scene appear to be slanting slightly up or down or to one side. Further, objects may appear larger or smaller than they really are. Normally these changes only cause a slight feeling of uneasiness for a period of a few hours to several days.
There's a lot of difference, however, between adjusting to corrective lenses on the ground and making the same adjustments while flying. When a pilot is using corrective glasses for the first time and experiences a sensation that the runway seems smaller and thus farther away than normal or that it appears slightly tilted to one side, the visual perceptions may cause enough false cues to jeopardize flight safety. The new spectacle wearer should be sure that he or she has completely adapted to their spectacles before flying with them.
The challenge of adapting to corrective lenses is compounded by bifocals. These lenses are prescribed for people who have a problem focusing their eyes on things close up. Bifocals for pilots should have the reading segment height set so that the top of the segment falls just above the highest instrument on the aircraft's instrument panel and just below the glare shield. Along with the usual visual perceptual adaptations due to single vision glasses, the new bifocal wearer must also learn new scanning techniques and head positions to properly use the bifocal reading segment.
One of the more common problems of learning to fly with bifocals is when the pilot mistakenly looks through the bifocals when flaring for a landing. This results in a blurry and slightly magnified view of the runway and, most likely, a "rough" landing. Another problem occurs when the pilot tries to focus on the instrument panel and inadvertantly looks through both the upper and lower parts of the bifocals causing blurred vision. Vertigo may be induced in a new bifocal wearer while flying under instrument conditions. Some pilots may move their head into various positions, trying to find the best position to use the bifocal segment to read the instruments and approach plates. This frequent head movement may cause vertigo. Trying to learn how to use new bifocals while flying under Instrument Flight Rules (IFR) is definitely not a good idea.
Pilots who are new bifocal wearers can do several things to help themselves adapt to this new way of looking at things. The most important task is for the pilot to spend a half-hour or so in the cockpit of a parked aircraft and practice various head positions and movements they'll need to use to see the instrument panels, approach plates, and switches.
It would be even better for the flyer to spend an hour or so in a simulator working with their new bifocals. Only after the pilot feels confident with their bifocals, should they try a familiarization flight with the bifocal. If possible, this familiarization flight should be done with a safety observer. Several visual and IFR approaches with landings should be made until the new bifocal wearer feels confident with their scanning techniques.
Another frequently overlooked factor affecting spectacle wearers is obtaining the proper frame adjustment. Helmets, headphones, and oxygen masks can play havoc with the spectacle frame and frequently the wearer may need one set of glasses adjusted for flying and another set adjusted for non-flying wear. The best way to obtain the most comfortable frame adjustment is for the flyer to bring their helmet, headphones, and/or oxygen mask as appropriate to the eye care professional where the spectacles were purchased and let the technicians fit the frames to their head while wearing their headgear.
Most people adapt to their new single vision glasses within several days if the change is not great. Bifocal wearers may take longer to adjust to their new prescription but that time can be shortened to a minimum if the patient wears the bifocals full-time and follows the ground practice techniques. Flyers should keep in mind the changes in visual perception that may occur with new glasses and they should exercise caution when flying until they feel confident with their new vision.