Think about a pirate. What comes to mind? You probably thought of a skull and crossbones flag, a three-cornered hat, parrots on shoulders, and almost invariably, of eye patches. For at least a century, pirates have been depicted in literature and history as swashbuckling men wearing the ubiquitous pirate eye patch. If you want to dress up like a pirate today, it’s likely you’ll include an eye patch as part of your costume.
Why on earth would pirates wear eye patches? It turns out the answer is rooted in brain science — and a quirk of how the human eye processes light.
Most people are probably familiar with rods and cones — the photoreceptors in the human retina that allow us to perceive light. Even though it’s located in the eyes, the retina is technically part of the brain, and so are the photoreceptors within in. How do photoreceptors actually work? It turns out they’re full of tiny channels, called opsins. Opsins are proteins, and they grip an even tinier chemical, call retinal, which is derived from Vitamin A.
When a photon of light enters your eye, it crashes into your photoreceptors. Sometimes, it hits some of the retinal that your opsins are gripping. When it does this, it literally knocks the retinal out of the opsin, like a person throwing a ball at some stacked bottles in a carnival game (or a pirate shooting a cannon ball at another ship). When the retinal gets knocked out, the opsin changes shape. The photorecector detects this change, and sends a signal to your brain saying “There’s light here.”
See the issue yet? Light is constantly knocking retinal out of your opsins. After you’ve seen a certain amount of light, shouldn’t you go blind, because all your retinal would have been knocked out? Thankfully, no. As long as you’re getting enough Vitamin A, your brain will keep creating more retinal and re-loading your opsins, so you can go on seeing in the dark.
There’s a catch, though. Refreshing your opsins with new retinal takes time. That’s part of why it takes your eyes a while to adjust to the dark after you’ve been in a bright area. The bright light has knocked out all your retinal, and it can take 20–30 minutes for your body to put it all back, so you can start to see in the dark again.
What does all this have to do with pirates? Imagine you’re a pirate captain on a ship at sea. You’re planning to attack a British naval vessel. To do this, you’ll need to manage tasks both above and below decks. Above decks, you might need to trim sails, direct the helmsman, and give orders to your compatriots to board the enemy ship. Below decks, you’ll need to direct the canon crews, assess damage if the enemy ship fires back, and more.
The lower decks of a ship at sea are dark, and the upper decks are very bright — especially with light reflecting off the water. Remember that it takes 20–30 minutes for the human eye to switch from dark to light conditions. If you’re a pirate in the midst of a battle and you’re constantly running back and forth between the above and below decks parts of your ship, you don’t have 20 minutes to wait each time you switch locations. When you venture below decks, you need to see what you’re doing right away.
What’s a pirate captain to do? Well luckily, humans come with two eyes. As long as you shield one eye from light, it will remain adapted to the dark, even if you’re in bright sun. It’s likely, then, that pirates wore eye patches so that they could keep one eye adapted to the dark all the time, and easily transition between the upper and lower decks of their ships during a heated battle, while still maintaining their dark vision.
Here’s how this would work. Let’s go back to our pirate attack scenario. Your vessel is now approaching the British ship, and you’re preparing to attack. You’re at the helm, and you’ve got an eyepatch on your right eye, keeping it in full darkness even as you stand in the sun. Your British quarry fires their cannons, and your ship is hit! You run to the the hatch, slide down a ladder to the below decks portion of your ship, and deftly switch your eyepatch to your left eye. Your right eye was always in the dark, because of the patch. Even though it’s dark below decks, you can still see perfectly out of your dark-adapted right eye.
You assess the damage, and determine that your attack can continue. Climbing back out of the hatch, you cover your right eye again, preserving its dark vision. If you need to go below decks a second time, it will be there and ready for you. You continue your attack, and in no time the British ship is yours!
Pirate eye patches likely had nothing to do with eye injuries (or pirate fashion). They were almost certainly a clever trick, taking advantage of a quirk of the human brain and visual system to give pirate captains an advantage in battle by preserving their dark vision for trips below decks.
Historical evidence to back up this theory is scant. But then, historical evidence about the specifics of early pirate life is scant in general. And if pirates were indeed using eyepatches to get an advantage over their adversaries, it’s unlikely they would go around advertising it. Some empirical studies, however, back up this theory. In 2007, Mythbusters tested the theory of pirate eye patches, and found it plausible, noting that eyepatches really did allow a person to adapt their vision to darkness instantly while moving around a ship.
There’s also contemporary evidence for the effectiveness of eye patches in battle. A 1995 military manual for the US Army instructs soldiers to “close one eye when around bright lights” in order to protect “dark adaptation during night operations.” During the Cold War, nuclear bomber crews were reportedly issued eye patches, so they could cover one eye and keep it working in case they were blinded by the flash of a nuclear explosion.
If pirates kept an eye covered all the time, wouldn’t they have zero depth perception? Not necessarily. While binocular vision helps a lot with depth perception, it’s only one factor which allows your brain to calculate depth. Others include the relative size of objects, textures, and the way that objects move across the retina. People who are missing an eye can still see well enough to drive and live normally — pirates with one eye covered could still have navigated their ships with ease, especially after years of practice.
If eye patches worked so well for pirates, why didn’t all mariners use them? It’s possible that they did. Heroic engravings of British naval heroes may have simply omitted eye patches, based on their association with disfigurement and the time period’s often-cruel treatment of the disabled. There is still vigorous debate about whether British naval hero Lord Nelson — who was blind in one eye — wore an eye patch.
It’s also possible that pirates wore eye patches and British officers didn’t because their crews were structured differently. Pirate ships were famously egalitarian and democratic. Pirate captains may have been expected to take on multiple roles in battle, working with crew members both above and below decks.
In contrast, British ships — with their hierarchical structure — may have had more rigid roles. A captain going below decks during battle might have been seen as unseemly — they may have been expected to stay at the helm and let their underlings handle things below the bilge line. If that’s the case, they would have had less need to preserve their dark vision during a battle, and less need for eye patches.
Nutrition may have played a role, too. Remember that retinal production depends on dietary Vitamin A. A well-nourish person adapts to the dark in 20–30 minutes. But those with vitamin deficiencies take far longer — or don’t adapt to the dark at all. The British Navy famously figured out the need to supply sailors with vitamins well before anyone else. British sailors were called “Limeys” in part because their rations included citrus fruit, to supply essential vitamins at sea and prevent deficiency diseases like scurvy.
If British sailors were better nourished than their pirate adversaries, they may have been able to function in dark conditions without the need for an eye patch. Alcohol, too, is known to have unusually strong effects on the visual system — including on dark adaptation. If pirates really drank as much rum as popular legend suggests, they have have been at an additional visual disadvantage versus their British adversaries, who mostly drank watered-down grog.
The next time you see an illustration of a pirate with an eye patch, remember that it’s unlikely they lost their eye in battle. They likely wore the patch as an early brain hack — to get a visual advantage in battle (or to compensate for the nutritional advantages of their enemies).
Now if only science could account for those parrots and long beards, we’d be set.