The classic image of a pirate is a man with a parrot on his shoulder, possibly sporting a peg leg. Additional accessories include a tri-corner hat or a bandanna, earrings, assorted pistols and daggers, and almost inevitably an eye patch.
Common sense and wisdom suggest that eye patches were worn by pirates who had lost an eye from a wound inflicted during battle, possibly during intense sword combat. There is, however, a better explanation: Pirates may have worn eye patches to have better vision when going below-deck.
Pirates and other seafarers would have had to frequently go back and forth between the bright light on deck, amplified by light reflected off the sea, and the darkness of the interior of the ship. There would have been very little light in the depths of a ship. Perhaps a candle would be used, but even then the holds would be relatively dark. By keeping one eye covered while on deck, a pirate could see easily when he went below by simply removing the eye patch. The covered eye would be pre-adjusted for night vision.
It takes the eye about 30 minutes to fully adjust from a light atmosphere to a dark one. This time is necessary for the pupil to dilate fully. A wider pupil allows more light to enter the eye, thereby improving low-light vision. The pupil contracts to prevent excessive light from entering the eye in bright conditions, which can make it hard to see and can also damage the eye.
This seems to be a credible explanation for why pirates would have worn eye patches, but is it true? The TV show MythBusters examined this very question and decided that it is plausible that pirates wore eye patches to help see in the dark, but they could not confirm or deny that this actually occurred. No accounts of this practice exist from early pirate records, and eye patch-wearing pirates may have first been mentioned in fictional works like Robert Louis Stevenson’s Treasure Island.
Regardless of whether pirates covered one eye for this reason, the practice has long been common in other settings. The US Federal Aviation Administration still recommends closing one eye if using a light inside the cabin at night to maintain night vision. So if you are headed out searching for buried treasure, take a lesson from fictional pirates and real pilots: Always wear an eye patch.