Refractive errors happen when the shape of your eye prevents light from focusing precisely on the retina, the light-sensitive tissue at the back of the eye. The most common cause is a mismatch between the length of the eyeball and the focusing power of the cornea and lens. Globally, myopia alone affects roughly 27% of the world’s population as of 2010, and that figure is projected to reach 52% by 2050.
How the Eye Focuses Light
In a normally shaped eye, light passes through the cornea and lens, which bend it so it converges into a sharp point directly on the retina. The cornea provides about two-thirds of the eye’s focusing power (around 43 diopters on average), and the internal lens handles the rest, adjusting its shape to fine-tune focus at different distances.
A refractive error occurs when this system is out of balance. The eyeball might be too long or too short, the cornea might be too steep or too flat, or the lens might not bend light correctly. Any of these structural mismatches shifts the focal point in front of or behind the retina, producing blurry vision. Among all the eye’s optical components, axial length (the front-to-back measurement of the eyeball) is the single biggest determinant of whether you end up nearsighted, farsighted, or somewhere in between.
What Causes Each Type
Myopia (Nearsightedness)
Myopia develops when the eyeball grows too long relative to its focusing power, so light converges in front of the retina instead of on it. Distant objects look blurry while close objects stay clear. In clinical terms, mild myopia ranges from about -0.50 to -3.00 diopters, moderate from -3.00 to -6.00, and high myopia is -6.00 or beyond.
The excess growth isn’t uniform. Myopic eyes tend to elongate more along the front-to-back axis than they widen, taking on a slightly more elongated shape compared to a normally proportioned eye. This altered shape also affects how the eye processes light in the periphery, which some researchers believe feeds back into further growth.
Hyperopia (Farsightedness)
Hyperopia is essentially the opposite problem: the eyeball is too short, or the cornea is too flat, so light hasn’t fully converged by the time it reaches the retina. The focal point falls behind the retina, making near objects blurrier than distant ones. A normal cornea has a central refractive power of about 43 diopters. In documented cases of progressive hyperopia, corneal power has dropped as low as 37 to 40 diopters, well below the range needed for clear focus.
Most hyperopia in children actually represents leftover infant farsightedness that the eye’s natural calibration process didn’t fully correct (more on that process below). Low-to-moderate hyperopia falls between +0.50 and +3.00 diopters, and high hyperopia is anything above +3.00.
Astigmatism
Astigmatism occurs when the cornea or lens is curved more steeply in one direction than the other, like a football rather than a basketball. Light entering the eye focuses at two different points instead of one, producing blurry or distorted vision at all distances.
The cornea is responsible for most astigmatism. The lens contributes a relatively constant amount throughout life, so changes in astigmatism over the years are driven mostly by changes in corneal shape. One factor is eyelid pressure: the upper eyelid presses down on the cornea, and as the eyelid’s tone decreases with age, the cornea’s vertical steepness diminishes. Sustained near work may also play a role. The inward pull of the eye muscles during close focusing can impose force on the cornea, gradually altering its curvature over time.
How Childhood Eye Growth Sets the Stage
Babies are born with a wide range of refractive errors, most of them mildly farsighted. Over the first year of life, a remarkable self-correcting process called emmetropization kicks in: the retina detects whether light is focusing in front of or behind it and sends growth signals that adjust the eye’s length accordingly. By around age six, this process has largely finished, and most children’s eyes have calibrated themselves to near-perfect focus, settling at roughly +0.75 diopters.
When this process goes wrong, it can fail in two distinct ways. Some children start with a refractive error too large for the system to correct, or their calibration mechanism simply doesn’t work well enough. These children arrive at age six still significantly farsighted, representing a primary failure of the system. Myopia, on the other hand, usually develops later. Most children who become nearsighted had normal vision at age six. Their eyes successfully calibrated early on but then kept growing past the ideal length during the school years and adolescence, a secondary failure of the same system.
Genetics and Heritability
Refractive error is highly heritable. Twin studies consistently estimate that 75% to 88% of the variation in people’s refractive status comes down to genetics. The largest genome-wide studies have now identified 449 genetic risk regions associated with refractive error, and gene analysis has highlighted 23 specific genes involved in eye development.
The genetic architecture is genuinely complex. Rather than a handful of genes with large effects, roughly 1% of all common genetic variants across the genome contribute small, nonzero effects. This means refractive error is shaped by thousands of tiny genetic nudges rather than a few big ones.
Interestingly, the genetic risk profile varies by ancestry. One of the strongest risk variants found in European populations, near the LAMA2 gene, shows virtually no signal in East Asian or African populations. Conversely, a variant in the PDE4B gene is significant only in East Asian populations, and another variant at a different location appears only in people of African descent. This means your ethnic background doesn’t just influence your likelihood of developing a refractive error; it shapes which specific genetic pathways are involved.
Environmental Factors
Genetics loads the gun, but environment pulls the trigger, particularly for myopia. Two lifestyle factors stand out in the research: time spent on close-up tasks and time spent outdoors.
Children who spend more than three hours a day on near work (reading, screens, homework) are nearly four times as likely to be myopic compared to children who do less than an hour. Even one to two hours of daily near work roughly doubles the odds. The mechanism appears to involve the signals the eye uses to regulate growth. Prolonged close focus creates a sustained pattern of defocus that pushes the eye toward continued elongation. Breaking up near work into shorter sessions seems to slow this process.
Outdoor time is the strongest known protective factor. Children who go outside only once a week are more than four times as likely to develop myopia compared to those who go out twice a week or more. The International Myopia Institute considers outdoor time the safest strategy for myopia prevention, and intervention studies have tested programs adding 40 to 80 minutes of outdoor activity during school days, with targets of at least 14 hours of outdoor time per week. The protective effect likely comes from bright natural light stimulating retinal signals that help regulate eye growth, though the exact mechanism is still being worked out.
Age-Related Changes and Presbyopia
Even people who had perfect vision their entire lives will eventually develop presbyopia, the gradual loss of near-focus ability that typically becomes noticeable in your early to mid-forties. Unlike other refractive errors, presbyopia isn’t caused by the shape of the eyeball. It’s caused by changes inside the lens itself.
The lens needs to be flexible to change shape when you shift focus from far to near. Throughout your life, the proteins inside the lens undergo slow chemical modifications: they oxidize, clump together, and become insoluble. This process is cumulative because lens proteins are never replaced. By age 60 to 65, most of the reactive chemical groups in lens proteins have been oxidized. The result is a lens that grows progressively stiffer decade by decade, losing the elasticity it needs to bulge into the rounder shape required for close-up focus.
This stiffening happens to everyone. The only variation is in timing and degree, which is why some people notice reading difficulty at 42 and others not until 50, but no one escapes it entirely.