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Pilots need a healthy visual system to be able to perform optimally during all phases of flight. Good visual acuity and color vision is needed to spot other airplanes while flying. Good depth perception is essential to make safe takeoffs and landings.
There are a few things that can happen to your vision when you go up in the atmosphere. As one ascends, the amount of available oxygen decreases. Any decrease in the amount of oxygen being delivered to the central nervous system affects the eye first.
The extraocular rectus muscles that move your eyes around in your head will get weaker with increased altitude and lead to uncoordinated movements and possible double vision. One’s ability to focus up close (accommodate) is also weakened at altitude, causing blurry vision and difficulty reading and performing other near visual tasks. When does all this start? At cabin altitudes as low as 10,000 feet.
Because of increased metabolic demands at night, these ill effects can become apparent as low as at 5,000 feet. Thus, it is no surprise that the FAA recommends that pilots use supplemental breathing oxygen at altitudes above 10,000 feet during daytime and above 5,000 feet at night.