system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people with poor vision instinctively squint their eyes? Why does this make their vision slightly more clear?
gpt
In addition to the points made by bubrahuicize, when you "stop down" (increase the f/# of) a lens (including your eye), you decrease the size of the aperture. Doing this increases the depth of field, as the part of the image nearer the center of the lens is the least affected by this lens. Or, to put it another way, the part of the image nearest the center is the least sensitive to focusing error.

By definition, nearsightedness or farsightedness is the inability to properly focus an image when the object is a certain range of distances from the viewer. Squinting - effectively, decreasing the diameter of the eye - will minimize the focusing error.