How we are able to see comes down to light entering the eye, which hits the retina (a light-sensitive layer of tissue at the back) and is then converted into electrical signals via special cells called photoreceptors. These electrical signals travel from the retina through the optic nerve to the brain, where they are then seen as images. During this process, it’s the muscles of the eye that controls where it will focus and how to react to certain sights by controlling the amount of incoming light. 

As our vision plays a major role in our cognitive abilities and like the muscles in the body, the eyes can be trained to be improved over time, for example seeing more details, reacting faster eye-hand coordination and so on. However, eye muscles - just like all other muscles - can get strained and overworked, which can often lead to injuries that, in the worst case, can prevent a player from doing what they do best.