Tuesday, July 5, 2011

PALs for pilots: cockpit Friendly Lenses

The answer is that we slowly lose our
ability to focus from the time we are
born. Like a complex auto-focus cam-
era, the human eye has a lens located
directly behind the pupil, surrounded
by a radial-shaped muscle called the
ciliary body. The muscle is innervated
by a complex of nerve connections
between the brain, the pupil and the
other eye.
When looking far away—beyond 20
feet—the lens is in a neutral position;
no focusing is required. As the eyes
look at objects closer than 20 feet,
focusing becomes necessary; the closer
the object, the greater the focusing
You can actually quantify the
amount of focusing demand at a
given distance. For example, at 16
inches (40 centimeters), +2.50 di-
opters of power is required.

For you mathematics buffs, by definition, +1.00 diopter takes infinite (beyond 20 feet, or 6 meters) light and focuses it at 1 meter. At 30 centimeters,
+3.00 is required; at 20 cm., +4.00 and so on. You can see how, as you
get closer, the power requirement increases significantly.
At birth, the eye can focus as close as 2 cm, but as we age, this distance
decreases. However, most of us don’t notice what’s going on until it affects
our lives, usually with difficulties reading normally. On one flight, you
can read the minimums box on the plate, on the next you can’t.
Why does all this happen? The crystalline lens, which does the
focusing, becomes more sclerotic and hardens as we age. That’s because
the most unique tissue in the body, the lens itself, is about 30 percent
protein, where all our other tissues are almost completely made of water.
Proteins denature with time, so the lens gets harder. It’s not the muscle
weakening. It’s the lens stiffening. Eye exercises won’t help, by the way,
so don’t waste your time and money on hocus-pocus schemes. This isn’t a
muscle problem, it’s a lens problem. You can’t make those poor muscles do
any more than they already are. So, it’s either off to the eye doc or the AME,
who then sends you to the eye doc. I personally recommend an eye exam
before your medical, just to avoid rais-ing the profile with your AME.

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