Michael H. Mittelman, O.D., M.P.H., F.A.A.O.
Most pilots realize their visual needs in the cockpit are quite different from those required to sit in an office or read the newspaper at home. The distances at which navigational aids and instrumentation are used differ from the customary 16-inch reading distance most glasses are prescribed for. Pilots are also exposed to physical and physiological forces that affect visual function in the aircraft. Unfortunately, when visiting the eye doctor pilots may fail to explain their unique flying environment adequately to the doctor. To help solve this often frustrating problem, it is important to become informed vision care consumers by learning the right questions to ask and the different types of glasses or contact lenses available. By working together with your eye doctor you can get a final product that performs well for you in the cockpit and results in good, comfortable, safe vision.
The Reading Prescription
As people approach age 40, their eyes begin to change. Because of these normal changes, most people over 40 require either reading glasses or bifocals. The reduction in ability to focus on near objects is called presbyopia and represents a normal change associated with aging.
Eye doctors (optometrists and ophthalmologists) are taught that most people use a near working distance of 16 inches. As pilots, our near working distance ranges from 24-36 inches in the cockpit, considerably different from the classic 16-inch working distance. It is essential that this difference in working distance is explained to the eye doctor, so he/she can lessen the power of the near vision prescription used while flying. The lessor power allows you to focus at greater distances and gives you a greater range of clear vision in the cockpit. It will allow you to easily view gauges, navigational aids, and approach plates. The shortfall of the lessor prescription is that, depending on your age and other related factors, you might be unable to focus things closer than about 18 inches. Consequently, many patients require two pairs of glasses, one for flying and one for more routine uses.
As a pilot, it is important to ensure that the glasses you receive produce the least amount of distortion and require minimal adaptation time. Bifocals that obstruct a persons gaze downward in all directions should be avoided at all costs. The "executive" style bifocal, a bifocal that has a line going across the entire lens, is a poor choice for a pilot, since it is twice as heavy as any other bifocal lens, and it can totally blur distant vision in downward gaze. It has been my clinical experience that the best lens for first time bifocal wearers is a lens called the Straight Top 25 or 28 (ST-25 or ST-28), also called "half-moon" bifocals. They allow pilots to see things up close when looking centrally, yet because they don't go totally across the lens, they allow pilots to look around them, permitting good downward visibility. Another alternative is the ST-35. This is a little larger, but it still allows for seeing around the bifocal portion of the lens. Progressive addition bifocals (bifocals without a line) can produce peripheral distortions and be difficult to get accustomed to. Because of this, these types of bifocals are not generally recommended as a first choice for flying. If you already wear progressive bifocals and have no problems with them, there is probably no reason to change, but as a first-time bifocal, they can cause some problems. Aerobatic pilots should avoid these types of lenses because distortions could create spatial disorientation problems.
The color of cockpit lighting can also affect your near vision. Red cockpit lighting requires more focusing power to see things clearly up close. This may require a stronger bifocal power. White or blue-green lighting is easier to focus in and requires less power. Make sure you tell your eye doctor what color lighting you are functioning in so he/she can attempt to duplicate it in the exam room. It is also a good idea to bring an approach plate or other type of publication or sectional with you to the eye exam so you can actually test your new flying prescription before using it in the airplane.
The Distance Prescription
There are three primary reasons people require glasses to see clearly at distances: nearsightedness (myopia), farsightedness (hyperopia), and astigmatism. As depicted in Figure 1, nearsightedness is the condition where the eye has too much light bending power built into it. People who are nearsighted see better up close than they do far away. You might say that their eyes have built in magnifiers. Hence, nearsighted people generally have no problems seeing things up close, no matter how old they are.
As shown in Figure 2, farsightedness is the condition where the eye has too little light-bending power. This is generally not a problem for those under 40 years old because the eye acts like an autofocus camera, and compensates for the condition by adding more power when needed. Hence, distance vision is exceptionally clear throughout the early stages of a farsighted person's life. As farsighted folks approach 40, they will require glasses for reading, and they may have difficulty focusing at distance, especially towards the end of the day. Many pilots fall into this category.
Unfortunately, farsighted patients who require a distance prescription to see clearly sometimes find that it hard to adjust to their first pair of glasses. People generally can adapt within a couple of weeks. Knowing you are a pilot who requires exceptional distance vision may prompt the eye doctor to cut your distance prescription back a little; this will help ease the transition into distance glasses and not hurt your eyes one bit.
Astigmatism is the condition resulting from the imperfect shape of the cornea and/or the lens. Instead of being shaped exactly round, like a baseball, an astigmatic surface is shaped more like an ellipse, or a football. As shown in Figure 3, this creates two primary focal planes for the eye. The result is blurred vision. The type of lens that corrects astigmatism is called a cylinder. In relatively high powers, cylindrical lenses can cause distortion. Be aware if you have a moderate to high amount of astigmatism, every time you change your glasses, you may require a week or so to adapt to the new glasses. Astigmatism can be associated with nearsightedness or farsightedness, or it can be independent of the two.
It is important to protect your eyes from both visible sunlight and potentially dangerous, invisible, ultraviolet light. Wearing either prescription or nonprescription sunglasses will help to maintain your ability to dark adapt for a night flight and make you more comfortable in the cockpit during daytime flying. With thousands of sunglasses on the market, it is easy to pick a style and color that best suits your needs as a pilot. Sunglasses that block 85% of the light will still enable you to see well on a bright day and view cockpit instruments, yet protect your eyes ability to dark adapt. Sunglasses come in varying colors. The lens color with minimal color distortion is neutral gray. Dark green sunglasses are also very effective. Blue-blocking sunglasses have an amber tint to them. The shortfall of amber lenses is that faint yellow lines can also disappear when viewing through some of these lenses. Remembering that runway obstructions and closed runways are marked with yellow "X's" might make you a little more cautious if you fly with blue blockers. The great advantage of the blue blocker is that by blocking the blues, it eliminates much of the scatter created by the shorter wavelengths of light. The result of this is an apparent increase in contrast on hazy days.
No matter what type of sunglass you purchase, you need to ensure that the glasses block both ultraviolet A (UVA) and ultraviolet B (UVB). The literature reports that UVB (250-310 nm) can possibly cause cataracts, primarily affecting the lens of the eye. UVA (320-400 nm) as been linked to possible changes of the retina and also is associated with common sunburn. It is a good idea have a UV-blocking coating placed on all of your glasses, both clear and tinted, to ensure that you have continuous protection from harmful ultraviolet rays.
The Federal Aviation Administration approves the use of contact lenses for pilots. It is important that you work closely with your eye doctor to select an appropriate contact lens to meet your needs. There are several types of contact lenses available to you. The most common type of contact lens prescribed in this country is the spherical soft contact lens, which generally will give you great vision and good comfort. These lenses are available in disposable, daily wear, and extended wear designs. Soft spherical contacts are also the lens of choice if you are acrobatically inclined, since they are the most stable on the eye. Soft contact lenses that correct for astigmatism, called toric lenses, offer a great alternative to glasses, when appropriately fit. The shortfall of these lenses, is that if they rotate on the eye, vision can blur. Depending on how much astigmatism you have, toric lenses may not afford you the consistently good visual acuity you require as a pilot. The other type of lens available to you is the rigid gas permeable contact lens. Also called "semi-soft" lenses, these lenses give you good clear vision and are sometimes used when a patient has too much astigmatism for correction with soft contact lenses. The disadvantage of rigid gas permeable lenses is that they can be difficult get used to, and in some situations, depending on their size, can slip off the cornea. With any type of contact lens daily wear is recommended. It is healthier to avoid sleeping in lenses (extended wear) unless absolutely necessary. If you do wear a lens on an extended wear schedule, ensure that you work closely with your eye doctor and seek regular care.
Going to the eye doctor should be a regular, enjoyable event, even if you think your vision is good. In addition to performing the necessary measurements for glasses, an eye examination should also include a comprehensive eye health check. It should include testing for glaucoma, checking for cataracts and other problems associated with the forward portion of the eye, and dilating the pupil, to evaluate the retina and other structures of the eye. Making the eye doctor aware of your unique needs as a pilot and being an informed vision care consumer ensures that you continue to receive the appropriate care necessary for good, comfortable, safe vision.
Note: Opinions expressed in this article are the private views of the author and are not to be considered approved by or to reflect the views of the Navy Medical Department, the U.S. Navy, or the Department of Defense.