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Keratometry is the foundation of contact lens fitting. This guide makes K-readings feel simple and exam-ready.
If you could only master one instrument for the NCLE, the keratometer would be it. The test leans on K-readings constantly—40+ questions touch corneal curvature, with-the-rule vs against-the-rule astigmatism, and base curve selection. Every fitting decision starts here.
The keratometer doesn’t just measure a number. It tells you how the cornea is shaped, what kind of lens will align, and whether a patient will need a toric design. That’s why exam writers love it—they can hide a lot of clinical logic inside a simple K-reading.
In this guide, you’ll learn to position the patient, align the mires, interpret steep and flat meridians, convert between diopters and millimeters, and recognize instrument limitations. You’ll also see how Bausch & Lomb and Javal-Schiotz styles differ and why the fictitious refractive index 1.3375 matters.
When you finish, you should be able to read any K-value, translate it into clinical decisions, and explain your reasoning out loud.
A keratometer measures the curvature of the anterior corneal surface by analyzing the reflection of mires projected onto the cornea. The cornea acts like a convex mirror. Steeper corneas create smaller reflections; flatter corneas create larger reflections. The instrument uses this relationship to calculate curvature.
You typically record two principal meridians, 90 degrees apart. These are the steepest and flattest curves and are reported in diopters (D) or millimeters (mm). The NCLE expects you to know both units and convert between them using the formula D = 337.5 / mm.
Manual keratometers require you to align the mires by adjusting dials. Automated keratometers capture and calculate instantly, but the interpretation remains the same. On the exam, you’ll typically see manual-style reading questions because they test understanding.
Keratometers use a fictitious refractive index of 1.3375, not the true corneal index. That detail appears on many exams, so memorize it.
A standard keratometer includes an eyepiece, mires, and adjustment dials that control horizontal and vertical alignment. The axis scale indicates the meridian of the reading. The doubling system splits mires so you can align them precisely.
Bausch & Lomb keratometers measure one meridian at a time; Javal-Schiotz models use a two-position drum to read both meridians. That difference affects your workflow, so know which instrument you’re using.
The joystick controls positioning and focus. The chin rest stabilizes the patient. The reticle or alignment target keeps your axis stable. Each component serves one goal: precise alignment of the mires.
[Image: Labeled diagram of keratometer components]
[Image: Keratometer mire alignment close-up]
A typical K-reading looks like 42.00 @ 180 / 44.00 @ 090. Always list the flat meridian first. The difference between the two values is the amount of corneal astigmatism. Here, 2.00 D of WTR astigmatism exists because the steep meridian is at 090.
If the steep meridian is at 180, it’s ATR astigmatism. If it’s at 045 or 135, it’s oblique. This classification appears repeatedly on the NCLE, often with lens selection questions.
Convert between diopters and millimeters using D = 337.5 / mm. A 45.00 D cornea equals 7.50 mm. Remember: steeper corneas have higher diopters but smaller millimeter values.
If readings are inconsistent, check for patient fixation. A wandering fixation point can distort mires and create false astigmatism.
Quality tip: always document both meridians, even if they are close. The NCLE expects full notation.
Keratometry drives base curve selection. For RGP lenses, you often start “on K” or slightly flatter. For soft lenses, K-readings help you decide if toric design is necessary.
It also helps identify irregular corneas. If mires appear distorted or impossible to align, you may be dealing with keratoconus or post-surgical changes. The NCLE expects you to know that standard keratometry has limits.
In practice, you’ll use K-readings alongside refraction and topography. But for the exam, focus on classic keratometry and what it tells you about lens fit.
Expect questions about mires alignment, the fictitious index 1.3375, and WTR vs ATR. You’ll also see base curve selection tied to K-readings.
Memory aid: “Steep at 90 = WTR, steep at 180 = ATR.” Say it every time you read a K-value.
Answer the questions, then check your reasoning.
What is the fictitious refractive index used in keratometry?
Answer: B. 1.3375
The keratometer uses a fictitious index of 1.3375 to account for both corneal surfaces.
K-readings are 43.00 @ 180 / 45.00 @ 090. What type of astigmatism is present?
Answer: A. With-the-rule
The steep meridian is at 090, which indicates WTR astigmatism.
Opterio provides targeted NCLE practice questions with detailed explanations so you can master keratometry and every other instrument.