Squinting causes two reactions that help you visualise the world around you in better detail. First, it changes the shape of our eye, allowing light to be focused better. Secondly, it decreases the amount of light that is allowed to enter the eye. Light coming from a limited number of directions allows that light to be more easily focused.
At its core, vision is just the perception of light by our brains.
The retina contains two different types of photoreceptors responsible for vision: rods and cones. When light strikes these cells, it reacts with visual pigments within them.
Cones are responsible for high resolution and colour. The highest density of cone cells reside in an area of the retina called the macula. In the center of the macula is an area known as the fovea centralis. The fovea only contain cones that are tightly packed together. No rods are present here. This highly dense area of cones gives us our greatest image resolution. As we focus our vision on something specific, like the words you’re reading now, the eye continually moves so it refracts the light coming from those words, directly on the fovea, leaving you with a detailed image.
When the eye is completely open, light waves from a wide range of directions are entering it. All of those waves are processed by all of the rods and cones in the different areas of your eye. By squinting, you are reducing the amount of light, and the number of incoming angles, that needs to be focused, making it easier to do so. It’s like trying to hear a specific person in a room filled with people talking. The unwanted noise drowns out the noise you actually want to focus on making it more difficult.
The shape of your eye’s lens and its ability to change shape, allows us to focus the light entering the eye, on the fovea. Should you be born with an abnormally shaped lens or eyeball, or your lens loses its elasticity (as can happen with age), its ability to focus light on the fovea is reduced. By squinting, we change the shape of our eye, ever-so-slightly. This helps the lens focus the light appropriately on the fovea.