What Is Myopia?
Myopia (nearsightedness) is a refractive error where distant objects appear blurry while near objects remain clear. In a myopic eye, light from distant objects focuses in front of the retina rather than on it. This happens because the eye is optically too powerful for its length, or too long for its optical power.
Myopia is corrected with minus (concave) lenses that diverge light rays, effectively pushing the focal point back onto the retina.
Types of Myopia
Axial Myopia
Axial myopia is the most common form. The eye's axial length is too long for its refractive power. A normal eye is approximately 24 mm long; each additional millimeter of length adds roughly 3 diopters of myopia.
Refractive Myopia
Refractive myopia occurs when the eye is a normal length but has too much refractive power. This can result from:
- A cornea that is too steep (high corneal power)
- A crystalline lens with increased refractive index (as in nuclear sclerotic cataracts)
- Anterior displacement of the lens
Classification by Amount
| Classification | Range | Characteristics |
|---|---|---|
| Low myopia | Up to -3.00 D | Most common, stabilizes in early adulthood |
| Moderate myopia | -3.00 to -6.00 D | Higher risk of complications |
| High myopia | Above -6.00 D | Significant risk of retinal detachment, glaucoma, macular degeneration |
Myopia Correction
Spectacle Lenses
Minus lenses are thinnest at the center and thickest at the edge. For higher prescriptions, opticians should consider:
- Higher-index materials to reduce edge thickness
- Aspheric designs to flatten the lens profile
- Smaller frame sizes to minimize the thick edge area
- Roll and polish the edge for cosmetic improvement
Contact Lenses
Contact lenses for myopia use the same minus power, but the effective power at the corneal plane differs from the spectacle prescription (vertex distance compensation applies above -4.00 D). Contact lenses also provide a wider field of corrected vision and eliminate spectacle magnification effects (minification).
Optical Effects of Minus Lenses
Minus lenses produce several optical effects that opticians should understand:
- Minification: Objects appear smaller through minus lenses (about 2% per diopter at standard vertex distance)
- Against motion: When moving the lens, objects appear to move in the same direction as the lens
- Base-up prism below OC: Looking below the optical center of a minus lens induces base-up prism (important for bifocal vertical imbalance)
- Ring scotoma: High-minus lenses create a ring of "missing" vision at the lens edge
Key Takeaways
- Myopia focuses light in front of the retina; corrected with minus (concave) lenses
- Axial myopia (long eye) is the most common form; ~3 D per mm of excess length
- The far point equals 1/prescription (e.g., -4.00 D = 25 cm far point)
- Minus lenses cause minification and against motion
- High myopia (above -6.00 D) increases the risk of serious eye diseases