Prescription for Success Eye Exams and Aviation 

MICHAEL H. MITTLEMAN, OD, MPH, FAAO

Sport Aviation - January 2000

Most pilots realize that their visual needs in the cockpit are quite different from those required to read Sport Aviation, but they may not be sure exactly what they are. One example is reading distance. Most glasses prescribed work best at the customary reading distance of 16 inches, but in an airplane the distance between the eyes and the instrument panel and charts is much different. Pilots are also subjected to physical and physiological forces that affect them in flight such as the different oxygen levels at altitude. 

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. In this prescription for success we’ll discuss the different things you should talk about with your eye doctor. 

Reading Prescription 
As people approach their 40th birthday their eyes begin to change, and they have difficulty focusing on near objects. This change, called presbyopia, represents a normal change associated with aging, and it often means people need reading glasses or bifocals. 

Eye doctors (optometrists and ophthalmologists) are taught that most people use a near working distance of 16 inches. In the cockpit, our near working distance ranges from 24 to 36 inches, considerably different from the classic 16-inch working distance. Explaining this difference to your eye doctor is essential so he or she can lessen the power of your near vision prescription used while flying. The lesser power allows you to focus at greater distance and gives you a greater range of clear vision in the cockpit. It will allow you to easily view instruments and charts. The shortfall of the lesser 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 pilots need two pairs of glasses—one for flying and one for everyday use. As a pilot, your glasses should produce the least amount of distortion and require minimal adaptation time. At all costs avoid bifocals that obstruct your gaze downward in all directions. “Executive” style bifocals, which have a line going across the entire lens, are a poor choice for pilots because they are twice as heavy as any other bifocal lens, and this style lens can totally blur distant vision in a downward gaze. 

In my clinical experience, the best lens for first time bifocal wearers is a 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 and, 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. It’s a little larger, but it still allows you to see 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’s 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. Make sure you tell your eye doctor what color lighting you are functioning in so he can attempt to duplicate it in the exam room. Whatever type of chart you use most—VFR or IFR—is a good idea to bring it with you to the eye exam so you can actually test your new flying prescription before using it in the airplane. 

Distance Prescription 
People need glasses to see clearly at distance for three primary reasons: nearsightedness (myopia), farsightedness (hyperopia), and astigmatism. Nearsightedness is 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.

Farsightedness is 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 auto focus 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. Farsighted people who require a distance prescription to see clearly sometimes find it hard to adjust to their first pair of glasses, but generally they adapt to them in a couple of weeks. Knowing you are a pilot who requires exceptional distance may prompt your doctor to cut your distance prescription back a little; this will help ease the transition into distance glasses and not hurt your eyes. 

Astigmatism results from the imperfect shape of the cornea and/or the lens, and it can be associated with nearsightedness or farsightedness, or it can be independent of the two. Instead of being shaped exactly round, like a baseball, an astigmatic surface is shaped more like an ellipse, or a football. This creates two primary focal planes for he 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. You may need a week or two to adapt every time your prescription changes.

Sunglasses 
Protecting your eyes from both visible sunlight and potentially dangerous, invisible, ultraviolet light is important. Wearing either prescription or non-prescription sunglasses helps to maintain your ability to “dark adapt” for a night flight and make you more comfortable during daytime flying. With thousands of sunglasses on the market, picking a style and color that best suits your needs as a pilot is easy. Sunglasses that block 85 percent of the light will 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. Neural gray colored lenses have minimal color distortion. Dark green sunglasses are also very effective. Blue-blocking lenses have an amber tint to them. Their shortfall is that faint yellow lines can “disappear” when looking through some of these lenses. Remembering that runway obstructions and closed runways are marked with yellow Xs should 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 sunglasses 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 nanometers—nm) can possibly cause cataracts, primarily affecting the lens of the eye. UVA (320-400 nm) has been linked to possible changes of the retina and is associated with common sunburn. It is a good idea to have a UV-blocking coating placed on all of your clear and tinted glasses to ensure that you have continuous protection from harmful ultraviolet rays. 

Contact Lenses
The FAA approves the use of contact lenses for pilots. Working closely with your eye doctor to select an appropriate contact lens to meet your needs is important. There are several types of contact lenses available, and the most common type prescribed in the United States 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. 

If you are aerobatically inclined, soft spherical contacts are your best choice because 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. Their drawback is that if they rotate on the eye, vision can blur. Depending on how much astigmatism you have, toric lenses may not give you the consistently good visual acuity you need as a pilot. A rigid gas permeable contact lens is another type available to you. Also called “semi-soft” lenses, they give you good clear vision and are sometimes used when a patient has too much astigmatism to correct with soft contact lenses. The disadvantage of rigid gas permeable lenses is that they can be difficult to get used to. In some situations, depending on their size, they can slip off the cornea. 

With any type of contact lens daily wear is recommended, even if you have extended-wear lenses. Don’t sleep with your contacts in 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, your eye examination should include a comprehensive eye health check. This 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. 

Michael H. Mittleman, OD, MPH, FAAO, is a member of the EAA Medical Advisory Council. The 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 or the Department of Defense.