Visual Deficiencies Night Myopia Hyperopia Astigmatism Presbyopia Vision in Flight Figure 12-1.  Effects of dimming cockpit lighting during night flight  to better see surrounding terrain. 12-2 be as dim as possible. The eye can then adjust for the outside  lighting conditions (ambient lighting) to see outside. The  dimmer the inside lighting is, the better you can see outside.  Hyperopia is also caused by an error in refraction. In a  hyperopic state, when a pilot views a near image, the actual  focal point of the eye is behind the retinal plane (wall),  causing blurred vision. Objects that are nearby are not seen  clearly; only more distant objects are in focus. This problem,  is referred to as farsightedness . At night, blue wavelengths of light prevail in the visible  portion of the spectrum. Therefore, slightly nearsighted  (myopic) individuals viewing blue-green light at night may  experience blurred vision. Even pilots with perfect vision find  that image sharpness decreases as pupil diameter increases.  For individuals with mild refractive errors, these factors  combine to make vision unacceptably blurred unless they  wear corrective glasses. Another factor to consider is “dark  focus.” When light levels decrease, the focusing mechanism  of the eye may move toward a resting position and make the  eye more myopic. These factors become important when  pilots rely on terrain features during unaided night flights.  Practicing good light discipline is very important and helps  pilots to retain their night adaptation. Keeping the cockpit  lighting on dim allows the pilot to better identify outside  details, unmarked hazards such as towers less than 200'  AGL, and unimproved landing sites with no hazard lighting.  A simple exercise that shows the effect of high versus low  light contrast would be to go out to a very dark road and  turn the dash board lights down very low or off and let your  eyes adjust to the ambient light level. Then, turn the dash  board lights up and note how the outside features disappear.  The same concept applies to cockpit lighting and being able  to see the surrounding terrain and obstacles.  [Figure 12-1] Special corrective lenses can be prescribed to pilots who  experience night myopia. An unequal curvature of the cornea or lens of the eye causes  this condition. A ray of light is spread over a diffused area  in one meridian. In normal vision, a ray of light is sharply  focused on the retina. Astigmatism is the inability to  focus different meridians simultaneously. If, for example,  astigmatic individuals focus on power poles (vertical),  the wires (horizontal) are out of focus for most of them.  [Figure 12-2] The eye automatically adjusts for the light level experienced.  During night flight, the cockpit and instrument lights should  This condition is part of the normal aging process, which  causes the lens to harden. Beginning in the early teens, the  human eye gradually loses the ability to accommodate for  and focus on nearby objects. When people are about 40 years  old, their eyes are unable to focus at normal reading distances  without reading glasses. Reduced illumination interferes with  focus depth and accommodation ability. Hardening of the lens  may also result in clouding of the lens (cataract formation).  Aviators with early cataracts may see a standard eye chart  clearly under normal daylight but have difficulty seeing under  bright light conditions. This problem is due to light scattering  as it enters the eye. This glare sensitivity is disabling under  certain circumstances. Glare disability, related to contrast  sensitivity, is the ability to detect objects against varying  shades of backgrounds. Other visual functions decline with  age and affect the aircrew member’s performance: The visual sense is especially important in collision  avoidance and depth perception. Due to the structure of the  human eye, illusions and blindspots occur. The more pilots  understand the eye and how it functions, the easier it is to  compensate for these illusions and blindspots.  Figure 12-3 shows the basic anatomy of the human eye and how it is like  • Dynamic acuity • Recovery from glare • Function under low illumination • Information processing