I never knew this! Via LiveScience, via Cognitive Daily:
Squinting reduces the amount of peripheral light coming into the eye so that a greater percentage of light comes from the center of the visual field… It’s wrong to to say that “‘squinting squishes the eyeball slightly to correct for a focus point that misses the mark.’ Although the lens does change shape, this is a reflex muscle action that can accompany (but is not the result of) squinting.”