We all have eyeballs and we use them in flying. Some of us wear glasses and some haven't reached that stage of maturity (but, it will happen). Let's talk about vision, vision as you age, night vision, depth perception and some of the newer procedures to correct your vision (radial keratotomy and laser correction).

We'll keep it simple, because I am a bone and joint kind of guy. Ancient and archaic standards state that normal vision is 20/20. This means that you see what you should at twenty feet. 20/100 means that you see things clearly at twenty feet that you should see clearly at one hundred feet. Obviously, 20/15 means you see better than normal and see things at twenty feet that normal people should see at fifteen feet.

The most common visual problem is myopia or a person is near sighted. For example, a person with 20/100 vision would be called near sighted or myopic. Far sighted would be the opposite. I won't cover all things, but I will add a word or two about astigmatism. This problem has to do with shapes being abnormal (a straight line appears curved).

The simple basics of vision (assuming your nerves, brain, etc are normal.........big assumption for Birddog pilots) are that light and an images pass through the cornea and lens and focuses on the retina. The cornea is the outside portion of the eye, in front of the lens, that you can touch. The retina is the back of the inside of the eyeball. For us who need simplicity, it is the screen where the image is projected.
If the image focuses on the screen, exactly, you have 20/20 vision.

The problem comes if you don't focus exactly on the retina (screen). A person is near sighted if he focuses in front of the retina (most common) and far sighted if he focuses behind the retina. There are several reasons or problems that can cause this. The three most common reasons for being near sighted are the shape of the cornea, shape of the lens (and ciliary muscle pull) and the length of the eyeball.

If the eyeball length is too long for the cornea and lens, the image will focus if front of the retina and the person will be near sighted. This would be one of the common inherited problems (where everyone in a family wears glasses).

The shape of the cornea is fixed and accounts for approximately three-fourths of the focus of an image. An abnormal curve can cause abnormal focus. If the cornea is not absolutely spherical, you get astigmatism (straight lines
looked curved).

The lens is flexible and is located behind the cornea. It accounts for about one-fourth of the focus and is attached to the ciliary muscles. They help with fine focus and accommodation. As we get older, the lens get less flexible and you cannot accommodate...................that is why you need the reading glasses after the age of forty. The muscles and cornea are the same, but the lens is stiff and less flexible.

Night vision is a function of the rods (remember the rods and cones). You aren't using the cones (color). A person who is 20/20 during the day is only 20/50 at night. There are less nerve endings to perceive images at night. The rods are very sensitive to oxygen and a decrease in oxygen greatly decreases night vision. Smoking has been proven to decrease night vision, due to the decrease in the oxygen carrying capacity of the blood. Get the point?

So, in summary, with all else normal, your corneal and lens (with ciliary muscles) focus an image on the retina. If the image hits the retina, you have 20/20 vision. If the image focuses in front of the retina, you are near sighted. As you get older, you may have good distant vision, but your lens gets less flexible and you usually need reading glasses. All devises (glasses, contacts, etc.) or procedures (keratotomy, lasik, etc.) have the same goal of getting the focus on to the retina.

CONTACT LENSES

Contact lenses are a way to change the focus of the image. They are a simple lens that lay outside of the cornea. They are made in three varieties. You can get two lenses that correct for distant vision. You can get one lens that corrects for distant vision and one that corrected for near (reading) vision. Your brain quickly sorts this out. You can get bifocal contact lenses. These are actually weighted at the bottom to try to keep the near vision area at the bottom.

RADIAL KERATOTOMY

This is an interesting procedure that started in the Russian Army. The Russian Army developed this technique so they would not have to put glasses on millions of new army recruits.

In this procedure, the cornea is reshaped using a series of 6 to 16 cuts. The length, position and depth of cuts are used to reshape the cornea and focus the image on the retina. The central portion of the cornea is not touched. There have been good results from this and it was "the rage" 15 years ago. Problems can be scar, seeing a "haze" and peripheral "sparkles" at night.

LASIK

The newest procedure is the laser shaping of the cornea. It is done by computer and microscopically changes the shape of the cornea. The actual amount of cornea taken off is about one-half the size of a hair. This new technique seems to have promise for the future.

DEPTH PERCEPTION

There are many things that go into depth perception. Binocular vision (two eyes) is always stated first. But, many other things are important. In actuality, many people with one eye have normal or near normal depth perception. The following things all contribute to depth perception:

1. Binocular vision

2. Size of object.....The closest of similar object (2 cars) will be bigger
Different objects give clues (if a car looks bigger than a house, it is closer)

3. Convergence......The road and telephone poles converge into the horizon.

4. Clear/dull or Bright/Blurr....Closure objects are sharper.

5. Motion Parallax.....Close objects move at different rate from distant objects.

6. Lights............At night, lights close to you are brighter that those far away.

All of these things are registered in your brain and you don't even have to think about them. A good example is landing at Wichita last summer. The runway was 250 feet wide and several miles long. Your depth perception and visual clues could easily get messed up.


Fly Safe! REX 79

Written for Birddog owners by Dr. Steve Frushour, IBDA Aero-Medical Officer