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Refraction: dioptres and reading a prescription

  • Topic

A spectacle prescription gives the lens power (sphere), the astigmatism correction (cylinder) and its direction (axis), plus extra power for reading.

Updated

See it in 3D: Cornea · Lens

What it is

Refraction measures the lens power that brings an image into focus on the retina. Power is measured in dioptres, the inverse of the focal length in metres. Myopia (short sight) is corrected with minus (diverging) lenses, hyperopia (long sight) with plus (converging) lenses, astigmatism with cylindrical lenses at a given axis, and presbyopia with an addition for near. Uncorrected refractive error is the leading cause of visual impairment in the world and the easiest to treat.

Key facts

  • Sphere: minus for myopia, plus for hyperopia, in dioptres.
  • Cylinder and axis: the amount and direction (0 to 180 degrees) of astigmatism correction.
  • Add: extra plus power for near vision in presbyopia.
  • Spherical equivalent: sphere plus half the cylinder.
  • Anisometropia: a significant difference in refraction between the eyes, a cause of amblyopia in children.

In clinical practice

  • A myopic eye is too long or too powerful, so the image falls in front of the retina; a hyperopic eye is too short.
  • Children can hide hyperopia by accommodating, which is why their refraction is measured with cycloplegic drops.
  • Colombian guideline thresholds for glasses in children depend on age: for example, from 3 to 6 years, myopia of 1 dioptre or more, hyperopia of 3 or more, or astigmatism of 1.5 or more.

Next step / Initial management

  • Reduced acuity that improves with a pinhole: refraction by optometry.
  • Children: refraction under cycloplegia; prescribe according to age, acuity, size of the error, strabismus and symptoms.
  • Follow-up of children with glasses: first review at 4 to 6 weeks, then every six months under 5 years and yearly after.
  • Sudden change in refraction in an adult: think of diabetes (glucose swings), cataract or keratoconus.

Test yourself

Think of the answer, then tap the question to see it.

Which lens corrects myopia?

A minus (diverging, concave) lens.

What does the 'add' on a prescription mean?

Extra plus power for near vision, used in presbyopia.

Why are children refracted under cycloplegia?

Because their strong accommodation can hide hyperopia; cycloplegic drops relax it and reveal the true error.

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Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.

Sources: Guía de práctica clínica para la detección temprana, el diagnóstico, el tratamiento y el seguimiento de los defectos refractivos en menores de 18 años (Guía No. 47) (Ministerio de Salud y Protección Social de Colombia, 2016); Hyperopia (StatPearls Publishing (NCBI Bookshelf), 2023); Astigmatism (StatPearls Publishing (NCBI Bookshelf), 2023); Presbyopia (StatPearls Publishing (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.