Why does squinting give me a headache?
Are you squinting like a pirate because you can’t see clearly? Or perhaps you’re trying to read your rates bill, which is both stressful and hard to read. In either case, tensing the muscles around your face and neck might cause a headache.
When you squint, your eyelid covers part of your pupil, which reduces the aperture down to about one millimetre, roughly what it would be in bright sunlight. That increases the range of objects that are in focus and helps you to see things that would otherwise be blurry.
In dim light, the opposite effect occurs as the pupil dilates, which increases the aperture. That lets in more light, but reduces the range of focus known as the “depth of field”. In poor light, reading is doubly difficult because the text is dim while you also have reduced focus.
In some ways, the eye resembles a camera with a lens that focuses light onto a receptor. In a digital camera, the receptor is a light-sensitive chip, but the eye is a little different because it has a retina with an area of greatest sensitivity called the fovea. In a camera, a shutter flips open to take a photo, while in the eye, your gaze flits around a scene.