What Is a Keratometer? How It Works and How to Choose One

What is a keratometer? It is an instrument — historically called an ophthalmometer — that measures the curvature of the front surface of the cornea and reports the axis and extent of corneal astigmatism. That is the whole job. It projects an illuminated target onto the cornea, measures the reflected image, and converts what it sees into two numbers per eye plus an axis. Everything else an eye care practice does with keratometry — fitting contact lenses, calculating intraocular lens power, checking whether refractive cylinder and corneal cylinder agree — runs downstream of those numbers.

This article covers what the instrument actually measures, the optics behind the reading, why two keratometers can disagree about the same eye, and how to decide between a standalone keratometer, a combined autorefractor/keratometer and a corneal topographer. If you have already made the category decision and want a model shortlist, our autorefractor buying guide is the commercial companion to this page.

What a keratometer measures — and what it does not

The National Library of Medicine's StatPearls review defines the keratometer as "an instrument used to measure the curvature of the anterior corneal surface and assess the extent and axis of astigmatism." Two things in that sentence do a lot of work: anterior, and curvature.

Anterior only. A keratometer reads the front surface of the cornea by reflection. It has no information about the back surface, so posterior corneal astigmatism is not in the reading at all — it is estimated by convention, not measured.

A very small zone. Keratometry samples a fixed chord of roughly 2–3 mm, described elsewhere as a 3–4 mm diameter ring, and it takes that reading from just four locations — two points on each of the two principal meridians. An adult cornea is about 11.7 mm across horizontally. The keratometer is therefore characterising the whole optical surface from a zone barely a quarter of its width, at four points.

A geometric assumption. The instrument assumes the cornea is a sphere or a spherocylinder. Real corneas are aspheric and radially asymmetric — they flatten toward the periphery, and after ocular surgery they may be frankly irregular. When the assumption holds, keratometry is fast, repeatable and entirely adequate. When it does not, the reading is still produced; it is just less meaningful. Knowing which situation you are in is the practical skill.

How a keratometer works: mires, doubling and the reflected image

Keratometry models the tear-film-covered cornea as a convex mirror. Project an object of known size at a known distance, and the size of the reflected image varies with the mirror's curvature. StatPearls gives the working relationship as r = 2ul/o, where r is the radius of curvature, o the object size, l the image size and u the distance. Measure the image, and you have the radius.

The complication is that the eye never holds still. Micro-movements shift the reflected image continuously, so measuring it directly is hopeless. The fix, which every keratometer since Helmholtz uses, is doubling: the optics deliberately produce two images of the mire and the operator aligns them until they just touch. Because both images move together when the eye moves, the alignment stays valid through the motion. It is an elegant piece of nineteenth-century engineering that survives unchanged inside modern automated instruments.

Helmholtz versus Javal-Schiotz — one position or two

The two classical designs differ in which variable they hold fixed.

• Helmholtz-type (variable doubling). The object size stays fixed and the amount of doubling is varied until alignment is achieved. The Bausch & Lomb instrument is the familiar implementation: four-aperture diaphragms with base-up and base-out doubling prisms produce two image pairs offset by 90°, so both principal meridians can be read from a single instrument position. This is the "one-position" keratometer.
• Javal-Schiotz-type (variable object size). The doubling is fixed — classically by Wollaston prisms exploiting birefringence — and the object size or its angle is varied instead. Reading both meridians requires rotating the instrument, which is why this design is called "two-position."

For a buyer in 2026 this is mostly context rather than a decision point: nearly every instrument you will be quoted is either a one-position optical keratometer or a fully automated unit. It matters when you are training staff on an older instrument you already own, or when a used unit's documentation calls a design by name and you need to know what technique it demands.

The 1.3375 problem: why two keratometers disagree about the same eye

This is the single most useful thing on this page, and it is the part spec sheets never explain.

A keratometer measures a radius, in millimetres. It reports a power, in diopters. Converting one to the other requires a refractive index, and the index used is not the cornea's real one. The true refractive index of corneal tissue is 1.376. Keratometers instead use a lower "keratometric" or effective index — conventionally 1.3375 in the United States, giving the familiar D = 0.3375/r — which fudges the anterior and posterior surfaces into a single number. The convention traces back to Helmholtz's original calibration.

The catch: the convention is not universal. 1.3320 is used in Europe, and instruments in the field publish other values too. Among the units we carry, the Ezer ERK-BH states its corneal power range "when cornea equivalent refractive index is 1.3375," while the Ezer ERK-770 states 1.337. Those are small differences that produce real, systematic offsets in the diopter column.

What to do about it: when you replace or add a keratometer, find the assumed index in the spec table before you compare readings between the old and new instrument, and before you compare a K value to one from an optical biometer or a topographer. If the numbers shift by a few tenths of a diopter on day one, the index is the first thing to check — not the instrument. Comparing the radius in millimetres sidesteps the issue entirely, because that is the quantity actually measured.

Measurement range and why older instruments need auxiliary lenses

A classical keratometer's native range runs roughly 36–52 D (6.5–9.38 mm), extendable to about 30–61 D with a −1.00 D or +1.25 D auxiliary lens over the objective. That limit is a genuine constraint on very flat or very steep corneas, and the auxiliary-lens workaround adds a step and a conversion table to the workflow.

Modern instruments have largely engineered the problem away. The Luxvision KR-800C publishes 35–68 D natively; the Ezer ERK-BH publishes 25.96–67.50 D. If your practice sees post-surgical or unusually steep corneas, native range is worth checking on the quote — but it is rarely the deciding specification any more.

What K readings do in a working lane

StatPearls lists keratometry's applications as intraocular lens power calculation, contact lens fitting, assessment and monitoring of astigmatism, and estimating refractive error where the media are hazy. Operationally, that translates into a handful of recurring uses:

• Contact lens fitting. Base curve selection starts from K. This is also where the small sampled zone bites hardest — a soft lens covers the entire cornea and beyond, while the keratometer has characterised the central few millimetres.
• Cross-checking the refraction. Corneal cylinder that disagrees sharply with refractive cylinder is a signal worth investigating rather than dispensing around.
• Pre-operative measurement. K values are a required input to IOL power formulas.
• A starting point when subjective refraction is difficult. Keratometry is objective and takes seconds.

For the clinical reasoning behind objective measurement in the pretest sequence, see our companion article on the importance of autorefractors. This page stays on the instrument and the purchase.

Four ways to get a K reading

Instrument	How it reads	Operator skill	Best fit
Manual optical keratometer	Operator aligns mires by eye and reads an internal scale	Highest — a learned technique	Teaching, a backup instrument, a lane that already has separate refraction, no-electronics simplicity
Handheld / portable keratometer	Automated capture in a hand-held body	Low	Bedside, wheelchair, pediatric, satellite sites, outreach
Combined autorefractor / keratometer (AR/K)	Automated; refraction and keratometry in one capture sequence	Low	The default main-lane instrument for most practices
Corneal topographer	Maps thousands of points across a wide corneal area; can output simulated K	Low to capture, higher to interpret	Specialty contact lens work, irregular and post-surgical corneas, surgical planning

One clarification that catches buyers out: "handheld autorefractor" and "handheld keratometer" are not the same instrument. The Ezer ERH-770 handheld autorefractor, for example, is a Shack-Hartmann aberrometer that publishes sphere, cylinder and axis — and no keratometry at all. If you need portable K specifically, confirm the keratometry specification is in the published table rather than assuming a portable refraction device includes it.

Keratometer versus corneal topographer

The difference is coverage, and it decides the instrument.

Keratometry gives you the two principal meridians from four points inside a 3–4 mm zone. A topographer characterises a much wider area, with Placido-based systems sampling along the mires at intervals as fine as one degree, and can derive a simulated K for continuity with your existing records. For a regular, roughly spherocylindrical cornea the two will broadly agree, and the keratometer is faster and far cheaper to put in a lane.

Where they part company is irregularity. As the clinical literature puts it plainly, K values used in IOL calculations "should not be measured with keratometry in eyes that have undergone keratorefractive surgery or are irregular for any other reason." A cornea reshaped by LASIK or PRK breaks the fixed-index assumption described above, and four sampled points cannot describe a surface that is no longer symmetric.

The practical conclusion for most practices is not "topographer instead of keratometer." It is that the AR/K handles the daily volume and the topographer earns its place when your case mix includes specialty lens fitting, post-surgical eyes or premium IOL planning. They answer different questions, and they sit at very different levels of complexity and investment.

The buying decision: standalone keratometer or an AR/K?

Most practices buying today choose a combined autorefractor/keratometer, and for most that is correct: one seat in the pretest lane, one instrument to train on, one printout, refraction and K captured in the same sequence. The autorefractor/keratometer category is where the volume is.

A standalone keratometer still makes sense in specific situations:

• You already have refraction covered. If a digital refractor or an existing autorefractor handles sphere and cylinder, you may only need K.
• Teaching and technique. A manual keratometer shows a trainee the mires, the tear film and the doubling principle directly. Automated instruments hide all of it.
• A backup that cannot go down. A purely optical instrument has no display, no board and no firmware. It works during a power cut and it does not become unsupported.
• Portability. A handheld keratometer goes where a 20 kg table-top cannot.

Against that: a standalone occupies bench space and a step in the workflow to produce one of the two numbers an AR/K gives you together, and a manual unit's readings depend on operator technique in a way an automated capture does not.

Specifications that actually matter when you compare

• Keratometry radius range (in mm) — the quantity actually measured, and the only one you can compare across instruments without worrying about the index.
• Corneal power range and the assumed index. Compare like for like. A range quoted at 1.3375 is not directly comparable to one quoted at 1.337.
• Corneal astigmatism range and increment. Increments of 0.05/0.12/0.25 D are typical on current automated units.
• Corneal diameter measurement — useful for lens sizing; ranges differ meaningfully between models.
• Contact lens base curve (CLBC) mode. If you fit lenses, this lets the instrument read the base curve of a lens directly. It is present on some units and absent on others, and it is easy to miss on a spec table.
• Peripheral keratometry. Some AR/K units add a peripheral K mode, which extends what the instrument can say about the cornea outside the central zone.
• Minimum pupil diameter (for the refraction side of an AR/K) — it determines how often you get a reading on a small or difficult pupil.
• Output and connectivity. A built-in printer, and whether the unit offers USB, RS-232C, Bluetooth or Wi-Fi. Note that a connector on a spec sheet is not the same as a working link to your records system — confirm what the connection actually delivers with your specific software.
• Footprint and weight. Depth is usually the binding constraint on an instrument table, not width.
• Service, spare parts and warranty. An instrument that measures in hundredths of a millimetre needs a support path when it drifts or a part fails.

Keratometry across the US Ophthalmic range

Every figure below is taken from the current product page for each model. Where a specification is not published, it is left blank rather than estimated.

Model	Type	Radius of curvature	Corneal power	Corneal astigmatism	Notes
Luxvision KR-800C	Manual optical keratometer	5.5–11 mm	35–68 D	Axis 0–180°	Internal reading scale, multi-directional joystick, single-position measurement; 300×280×510 mm, 18 kg (40 lb)
Ezer EZ-KR-1800	Handheld keratometer	6.5–9.5 mm	—	Axis deviation ±2°	Precision ±0.05 mm, resolution 0.01 mm, 0.03 s per measurement, CLBC mode, wireless infrared thermal printer; 240×90×60 mm
Ezer ERK-BH	Autorefractor / keratometer	5.0–13.0 mm (0.01 mm)	25.96–67.50 D at index 1.3375	0.00 to −15.00 D	K/R, REF, KER, CLBC and peripheral-K modes; corneal diameter 2.0–14.0 mm; 7″ tilting touchscreen; thermal printer with auto-cutter; 260×500×450 mm, 20 kg
Ezer ERK-9000 A	Autorefractor / keratometer	5.0–10.2 mm (0.01 mm)	33.00–67.50 D	0.00 to −15.00 D	Corneal diameter 2.0–12.0 mm; minimum pupil 2.0 mm; 8″ colour TFT touchscreen; internal thermal printer with auto paper cut; motorized chinrest; 275×525×440 mm, 18 kg
Ezer ERK-770	Autorefractor / keratometer, full automatic	5.0–10 mm (0.01 mm)	33.75–67.50 D at index 1.337	—	Corneal diameter 2.0–15.0 mm; minimum pupil 2.0 mm; 10.4″ touchscreen; USB, RS-232C, Bluetooth, Wi-Fi; 514×284×465 mm, about 20 kg

A few things worth drawing out of that table. The ERK-BH is the only unit here that publishes both a contact lens base curve mode and a peripheral keratometry mode, and it has the widest radius range at 5.0–13.0 mm — relevant if you fit lenses or see flatter corneas. The ERK-9000 A trades some of that range for a larger 8″ display and a motorized chinrest. The ERK-770 is the connectivity option. And the KR-800C is the one instrument on the list that will still work when the power is out.

Ezer is US Ophthalmic's own brand, and every unit above is supported by our in-house technical and spare-parts department with US warranty and service — which is the part of a keratometry purchase that only shows up two years later, when something needs adjusting.

What to confirm before you sign

• Which keratometric index the instrument uses, if you will be comparing readings with an existing unit or a biometer.
• Whether CLBC mode is included, if contact lens fitting is part of your practice.
• The printer consumable — roll width and how you reorder it.
• What the data interface will actually deliver with your specific records software, in writing.
• Your instrument table's usable depth against the unit's depth, not its width.
• The warranty term and exactly which assemblies — chinrest motor, printer, touchscreen — are covered.
• Current availability and lead time on the specific model, which moves.

Frequently asked questions

What is a keratometer used for?

It measures the curvature of the front surface of the cornea and reports the axis and extent of corneal astigmatism. Those readings are used as inputs to contact lens base curve selection, intraocular lens power calculation, monitoring of astigmatism over time, and estimating refractive error when the ocular media are hazy.

What is the difference between a keratometer and an autorefractor?

An autorefractor measures the refractive error of the whole eye and reports sphere, cylinder and axis. A keratometer measures only the curvature of the anterior cornea. A combined autorefractor/keratometer (AR/K) does both in one capture sequence, which is why most practices buy the combined instrument rather than two separate ones.

How accurate is a keratometer?

Current automated instruments resolve the corneal radius to 0.01 mm, and the Ezer EZ-KR-1800 handheld publishes a precision of ±0.05 mm with axis deviation within ±2°. The larger source of variation is not the instrument's resolution but its assumptions: keratometry samples four points inside a 3–4 mm zone and models the cornea as a sphere or spherocylinder. On a regular cornea that is a good model. On an irregular or post-surgical cornea it is not, and topography is the appropriate instrument.

Why do two keratometers give different diopter readings for the same eye?

Because the diopter value is calculated from the measured radius using an assumed refractive index, and that index is a convention rather than the cornea's true value of 1.376. Instruments in the United States conventionally use 1.3375; 1.3320 is used in Europe, and other values appear in the field — the Ezer ERK-BH states 1.3375 and the ERK-770 states 1.337. Comparing the radius in millimetres avoids the discrepancy entirely.

Do I need a keratometer if I already have a corneal topographer?

Most topographers can output a simulated K, so the reading is available. Practices still keep a keratometer or an AR/K in the pretest lane because it is faster, cheaper to place, and captures K alongside refraction in one patient seating. The topographer earns its place on the cases keratometry cannot describe: irregular corneas, post-refractive-surgery eyes and specialty lens fitting.

Is there a handheld keratometer?

Yes. The Ezer EZ-KR-1800 is a handheld corneal curvature instrument measuring 240×90×60 mm with a single measurement time of 0.03 s, a CLBC mode and a wireless infrared thermal printer. Be aware that a handheld autorefractor is a different instrument — the Ezer ERH-770, for example, is a Shack-Hartmann aberrometer and does not publish keratometry.

What is a normal K reading?

Keratometers are built around the range human corneas actually occupy: a classical instrument's native range is about 36–52 D (6.5–9.38 mm), extendable with auxiliary lenses, and current automated units publish wider ranges — 25.96–67.50 D on the Ezer ERK-BH and 35–68 D on the Luxvision KR-800C. Interpretation of an individual reading belongs with the examining clinician.

What is the difference between a one-position and a two-position keratometer?

A one-position instrument, following the Helmholtz principle with fixed object size and variable doubling, reads both principal meridians without being rotated — the Bausch & Lomb design is the classic example. A two-position instrument, following the Javal-Schiotz principle with fixed doubling and variable object size, must be rotated to read the second meridian. One-position is faster and is what most manual instruments in current use are.

Sources

Optical principles, sampled zone, keratometric index, measurement range and limitations: Keratometer, StatPearls, National Library of Medicine; Keratometry and Topography (clinical reference). Corneal dimensions and true refractive index: Anatomy of the Cornea. All model specifications are taken from the current US Ophthalmic product pages linked above and were verified on 7 August 2026. Request a quote for current configurations, availability and lead times.