Eugene Vishnya/Shutterstock
Stein Boddington
Sydney, Australia
The clarity of the image focused on the retina depends on the correct working of several elements, some of which can be modified by reducing the size of the pupil, aka the hole in the middle of the iris. Scrunching up your eyes, or squinting, does this by partially covering some of the pupil with your eyelids.
If your view is blurry, whether because the lens has an issue focusing or there is some other problem concerning the pathway the light takes, simply making the aperture smaller will improve the image quality. But by blocking some light, it does make the image dimmer.
Using our normally parallel eyelids to do this makes more of a narrow slit, so there is still some blurring in the horizontal direction, but it does improve the image.
Another reason for an unclear image is distortion in the surface of the cornea, the tough, transparent, exterior window of the eye. If a crease, wrinkle or other issue in the cornea is situated at the top or bottom of the pupil, then it can be eliminated by the eyelid coming up or down to cover it, leaving an undistorted, quality corneal surface to transmit a good image to the retina. Such problems are commonly caused by the cornea drying out along a line where the lids don’t perfectly close during sleep.
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Scrunching can slightly alter the shape of the eye, bringing the focal point of incoming light closer to the retina
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Control of the upper lid is better developed by the muscles used to blink and shut our eyes. Squinting brings in a few more muscles and forces the lower lid up, which is otherwise difficult.
Marthinus Roos
Elgin, Moray, UK
Several contributors have suggested that vision improves when we scrunch up our eyes because the eyelids obscure part of the pupil and thereby improve focus. This pinhole-like effect undoubtedly plays a role in sharpening the image.
However, these explanations overlook the fact that, in practice, people don’t simply lower their upper eyelids – they actively scrunch their eyes. This involves strong contraction of the eyelid muscles, which is quite different from a gentle half-closing of the lids.
The key point is that such forceful contraction can slightly alter the shape of the eye, bringing the focal point of incoming light closer to the retina.
Short-sighted readers can test this for themselves by comparing their vision when they merely half-close their eyes with their vision when they fully scrunch them up.
If the latter produces a noticeably greater improvement, it suggests that mechanical deformation of the eye and the pinhole effect of the lids contribute to the clearer image.
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