Here’s how the eye works, light passes through your pupil and then the lens which focuses the light for the retina located in the back of your eye. It is in the retina that special cells interpret the light and translate them into signals sent to your brain where the image is interpreted and bam you have vision. With age comes the hardening of the lens and the ever increasing inability to see clearly.
The lens becomes unable to focus the light. Squinting actually changes the shape of a person’s eye. It helps the light focus correctly in the retina. The act of squinting also decreases the amount of light that can travel through the eye.
When a lens is misshapen, the light that passes through the lens is deflected incorrectly and misses the focal point; the farther the light rays are from the center of the lens, the more they are deviated from the focal point. By limiting the rays of light that come in through the bottom and top of the pupil, squinting allows rays to pass closer to the center of the lens, thereby creating a more focused image.