Why pirates cover one of their eyes?

I always wonder why pirates cover one of their eyes with some piece of cloth.

They always use black cloth to cover their eye and don’t use any other colour. They look very interesting with one of their eyes covered with some cloth piece.

The reason behind this covering is very simple. We lose our eye-sight for some time when we move from an area of brightness to an area of darkness.

So, they cover one of their eyes in order to make it ready for looking in the darkness present below the deck.

http://pixabay.com/en/italy-pirate-sculpture-corsair-206958/