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How do structural differences in the human eye cause nearsightedness and farsightedness?

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Diagram of a horizontal section of the human eye showing labeled parts including the cornea, iris, lens, retina, sclera, optic nerve, and blood vessels, with internal structures and muscles visible. — human eye A horizontal cross section of the human eye, showing the major parts of the eye, including the protective covering of the cornea over the front of the eye.

Nearsightedness (myopia) occurs when the eye focuses images in front of the retina, causing distant objects to appear blurry. This often happens when the eyeball is longer than normal. Farsightedness (hyperopia) occurs when the eye focuses images behind the retina, making near objects appear unfocused, though distant objects may be clear. This typically happens when the eyeball is shorter than normal.12

In both conditions, the cornea and lens of the eye do not refract light correctly to focus a clear image directly onto the retina. Corrective lenses are designed to adjust the light's focus, either by supplying additional convexity for farsightedness or by altering the focal point for nearsightedness. Astigmatism, a related condition, involves a nonuniform curvature of the cornea or lens, causing blurred vision at all distances.3425