1. What Is Blur, Exactly?
Your eye focuses light onto the retina, the thin layer at the back that senses light. When the eye is focused on something, light spreading out from one point on it gets gathered back into one point on the retina. When the focus is off, that light does not gather into a point. It lands as a small disc instead [1]. That disc is the blur you notice.
Letters are made of thin lines. If every point of every line turns into a disc, neighboring lines overlap and the letter turns to mush. So reading blurry print is a fight against the size of those discs. Squinting helps because it makes the discs smaller.
2. How Does a Smaller Opening Shrink the Blur?
Light from one point on a page enters your eye as a whole bundle of rays. When the eye is out of focus, those rays do not meet in one point on the retina, and they spread into a disc instead. A smaller opening blocks the skewed rays that come in near the edge of the pupil, so the disc gets smaller [2]. As one optometry review puts it, when light passes through a small pinhole or pupil, the unfocused rays are blocked and mostly focused light lands on the retina [1].
Photographers know the same idea as the camera's aperture: a smaller aperture keeps more of a scene sharp. For blur that comes from a focusing error, the size of the blur disc grows in proportion to the width of the opening, according to the pupil-size review [2]. That is a simplified rule that leaves out the shape of the opening and the eye's other flaws. If your pupil is about 4 millimeters wide and the opening you look through is about 1.2 millimeters, the disc becomes roughly 0.3 times as wide, or about one third. That number is our own simple calculation, not a measurement from a study, and a real eye is messier than this picture.
Your pupil already does this on its own. Pupil size changes with lighting, effort to focus, age, and other things [2]. Squinting adds a second, adjustable opening: the gap between your eyelids.
3. What Do You Lose When You Squint?
Two things. The first is brightness. A smaller opening lets in less light, so the image gets dimmer. Optometrists note that a pinhole that is too small reduces the light reaching the retina [1].
The second is a surprise: an opening can be too small. Light behaves like a wave, and when a wave squeezes through a very narrow gap, it bends and spreads at the edges. This is called diffraction. Diffraction blurs the image again and takes away fine detail [1]. So shrinking the hole helps at first, then starts to hurt.
That means there is a best size. A clinical study of pinhole sizes, as described in the optometry review, found that holes between 0.94 and 1.75 millimeters worked best, and most pinhole cards sold for eye exams have holes of about 1.2 millimeters [1]. A review of the optimal pupil size reaches a similar kind of answer. For a well-focused eye in bright light, the best pupil size is around 2 to 3 millimeters, and in dim light the best size grows to about 6 millimeters, because dim light makes enough light more important than a tiny opening [2]. The review's short answer to "what is the best pupil size?" is "it depends" [2]. In other words, squeezing harder does not always help.
4. Why Do Eye Doctors Use a Card With Tiny Holes?
An optometrist's pinhole occluder is a dark card with many small holes, usually about 1 to 2 millimeters wide. It is cheap and simple, and it can even be made by punching holes in dark card [1]. A patient reads the eye chart through the holes. If the letters get clearer, the blur was probably caused by the kind of focusing problem that glasses can fix. If the letters do not get clearer, something else may be going on in the eye, and the doctor looks further [1].
So the card is a quick test that gives the doctor a clue about whether a focusing problem is involved. It is a tool for the doctor, not a way to treat anyone. Seeing better through a pinhole is not a diagnosis, and a doctor is the one who decides what is going on.
The same review points out that the gain happens only while you look through the small opening [1]. When the holes go away, the focusing problem is still there.
5. Does Squinting Fix Your Eyesight?
No. Squinting works around blur for a moment, and it does not change the shape of your eye or the focusing power of your lens. The cost is a dimmer, narrower view, and squinting for a long time can make your face and eyes tired.
If you find that you often need to squint to see the board at school or the road signs, or if your eyes feel tired after reading, that is worth telling a parent or another trusted adult so you can get an eye exam. Glasses or contact lenses fix the blur without making the world darker. Squinting is a good clue that something may need checking, not a long-term plan.
6. Where Else Does a Tiny Hole Make a Picture?
A very small hole in a dark space can project a whole picture of the outside scene on the opposite wall, upside down and flipped left to right. This is called a camera obscura, which is Latin for "dark chamber" [3]. Your own eye works somewhat like one: light enters through an opening, the pupil, passes through a lens, and forms an upside-down image on the retina [3]. People have also used a camera obscura to study eclipses without looking at the sun [3].
Squinting is a rough, homemade cousin of the same trick. It does not form a picture. It simply narrows the opening so the blur disc shrinks.
7. How Can You Try the Pinhole Effect Safely?
With an adult nearby, try this in a bright room, for a short time. Slowly bring a page of text toward your face until the letters start to blur. Do not bump your nose. Then make a small ring with your thumb and first finger and look at the text through the ring. Some people notice the letters look a bit sharper, and some do not. Make the ring smaller and see whether the page gets dimmer. Stop if your eyes feel tired.
Never look at the sun or any very bright light, with or without a pinhole. If you want to watch an eclipse, use a method that does not involve looking at the sun, such as projecting its image through a small hole onto a card.
Here are questions to explore next. Why would a bigger pupil in dim light make blur worse? How small can a hole get before diffraction takes over? The optometry review [1] and the pupil-size article [2] are good places to start. This page explains how things work. It is not medical advice. If something about your vision worries you, ask an eye doctor.