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What Is Manual Keratometry?

Manual keratometry is a clinical measurement of the anterior corneal curvature using a keratometer that is operated by the examiner. The instrument projects targets called mires onto the cornea, and the clinician aligns reflected images using knobs and prisms. The final reading estimates corneal power and astigmatism over a small central zone, typically around 2 to 3 millimeters. Manual keratometry is used for contact lens fitting, assessing astigmatism, and supporting cataract surgery measurements.

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What Is Manual Keratometry?

Manual keratometry is a clinical measurement of the anterior corneal curvature using a keratometer that is operated by the examiner. The instrument projects targets called mires onto the cornea, and the clinician aligns reflected images using knobs and prisms. The final reading estimates corneal power and astigmatism over a small central zone, typically around 2 to 3 millimeters. Manual keratometry is used for contact lens fitting, assessing astigmatism, and supporting cataract surgery measurements.

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How Manual Keratometry Works

The cornea behaves like a convex mirror, so the size of a reflected target changes with corneal steepness. In a manual keratometer, the examiner focuses the mires and adjusts controls until specific doubled images just touch or overlap. The drum or scale reading is then converted into a radius value and a diopter value for each principal meridian. Two readings, steep and flat, are used to describe corneal astigmatism and its axis.

What It Measures and What It Misses

Manual keratometry measures the anterior corneal curvature in the central optical zone and assumes the cornea is relatively regular. It does not directly map peripheral cornea, posterior cornea, or corneal thickness. Irregular corneas can distort mires and reduce accuracy, which is why topography or tomography may be needed. Tear film instability can also change the reflection and affect repeatability.

Common Uses

Manual keratometry remains a practical tool in many clinics.

  • Estimating corneal astigmatism and axis
  • Choosing starting parameters for contact lens base curve selection
  • Providing keratometry inputs for intraocular lens calculations

It may be repeated over time to track corneal changes from disease or contact lens warpage.

Limitations and Sources of Error

Errors can come from poor alignment, unstable fixation, eyelid interference, and dry eye that disrupts mire clarity. Recent contact lens wear can temporarily alter corneal shape, especially with rigid lenses, so a lens break may be recommended before measurement. Manual keratometry also samples only a small central area, so it may miss localized irregularity. When results look inconsistent, clinicians often repeat readings and compare with corneal topography.

FAQs on Manual Keratometry

Is manual keratometry still used if you have corneal topography?

Yes. Many clinics still use manual keratometry as a quick measurement and as a cross-check against other devices. Topography provides broader mapping, but keratometry can still be useful for central curvature and routine workflows.

Does manual keratometry measure the whole cornea?

No. It mainly measures a small central zone of the anterior cornea, usually around 2 to 3 millimeters. It does not directly measure posterior curvature or thickness.

Can dry eye affect manual keratometry?

Yes. An uneven tear film can distort the reflected mires and lead to variable readings. Treating the ocular surface and repeating the measurement can improve reliability.

Why do the numbers differ between keratometry and my glasses prescription?

Keratometry describes corneal curvature, while your glasses prescription describes the overall refractive power of the entire eye. The lens inside the eye and eye length also affect your prescription, so the numbers are not expected to match.

References

Keratometry. Cleveland Clinic. https://my.clevelandclinic.org/health/diagnostics/keratometry. Date Accessed February 4 2026.

Keratometer. StatPearls (NCBI Bookshelf, National Library of Medicine). https://www.ncbi.nlm.nih.gov/books/NBK580516/. Date Accessed February 4 2026.

What Do Astigmatism Measurements Mean? American Academy of Ophthalmology. https://www.aao.org/eye-health/ask-ophthalmologist-q/what-do-astigmatism-measurements-mean. Date Accessed February 4 2026.

Corneal Topography. EyeWiki (American Academy of Ophthalmology). https://eyewiki.org/Corneal_Topography. Date Accessed February 4 2026.

Corneal Topography. StatPearls (NCBI Bookshelf, National Library of Medicine). https://www.ncbi.nlm.nih.gov/books/NBK585055/. Date Accessed February 4 2026.