Ezer ERK-BH Autorefractor Keratometer: Full Specs and Practice Fit

Most write-ups of the Ezer ERK-BH autorefractor stop at two numbers: a dioptric range of −30.00 to +25.00 D and a corneal-radius range of 5.0 to 13.0 mm. Both are real, both are published, and neither tells you what you actually need to know before an autorefractor keratometer goes on a bench in your pretest room. This page is the layer underneath the headline: every specification Ezer publishes for the instrument, how its measurement envelope really compares with the other six ERK models we carry (the answer is not the one the headline numbers imply), the arithmetic that explains why it matters, and the four things worth getting in writing before you sign.

If you want the cross-brand shortlist instead, our 2026 ARK comparison ranks seven machines, and the autorefractor buying guide covers the category decision — standalone versus combination, table-top versus portable. What follows assumes you have already made that decision and are now looking hard at one instrument.

The five measurement modes, and what each one is for

Ezer publishes five named modes on the ERK-BH, and they are worth reading as a list of jobs rather than a feature count:

K(P) is the one that repays attention, and the reason is a limitation of keratometry itself rather than of this instrument. In StatPearls' reference article on the keratometer, Gurnani and Kaur state that keratometry is performed "usually across a fixed chord length of 2 to 3 mm" and that keratometers "take into account only the central curvature of 3 to 4 mm." A central K pair is a description of a small disc in the middle of the cornea. Peripheral fixation lamps are how an instrument in this class samples outside it, and Ezer publishes two lighting modes for them, AUTO and MANU.

Alongside the five modes, Ezer lists a set of secondary functions on the ERK-BH: a SIZE mode for measuring corneal diameter, pupil size and the diameter of a contact lens without removing it; an ILLUM mode that directs light into the pupil, which Ezer lists for inspecting the surface of a contact lens; a retro-illumination mode for hard contact lenses, with an enhanced display that shows ten images and results together; an IOL setting for eyes where a standard refraction returns an error reading; a CF (comparison of vision) mode; near-vision simulation; and a freeze mode in which corneal, pupil and hard-contact-lens diameters are measured by touching and dragging on screen.

The full published specification

Every figure below is from the specification table Ezer publishes on the ERK-BH product page. Nothing here is estimated.

Two notes on the catalogue itself, stated rather than smoothed over. The monitor row on the product page reads "7in (1778 mm)"; seven inches is 177.8 mm, so the metric figure carries a decimal-place slip. The same slip recurs on the ERK-H page, whose monitor row reads "5.7 inch (1448 mm())" — 5.7 inches is 144.8 mm. And the dimensions row opens with a stray character before "Approximately." Neither affects the instrument — the imperial and metric figures agree everywhere else, 20 kg is 44 lb, and 260 × 500 × 450 mm converts to the 11 × 20 × 18 in also shown — but if you are building a spreadsheet from vendor pages, these are the kind of transcription slips worth catching.

The measurement envelope: two models share it, five do not

US Ophthalmic carries seven Ezer ERK models. Read all seven specification tables side by side and the line splits cleanly in two on measurement range — and the split is not where the headline numbers suggest.

Sphere. The ERK-BH publishes −30.00 to +25.00 D. So does the ERK-9100 — the same figures, at the same 12 mm vertex distance. The ERK-H, ERK-9000 A, ERK-9200 and ERK-5400 A all publish −25.00 to +22.00 D, and the ERK-770 publishes −20 to +20 D.

Corneal radius. The same two models pull away again, and by more. The ERK-BH and the ERK-9100 both read corneal radii from 5.0 to 13.0 mm. The ERK-H, ERK-9000 A, ERK-9200 and ERK-5400 A all publish 5.0 to 10.2 mm; the ERK-770 publishes 5.0 to 10.0 mm.

The consequence is easiest to see in dioptres. StatPearls gives the standard conversion a keratometer performs as D = 0.3375 / r, with r in metres. Run the flat ends of those ranges through it:

The arithmetic is self-checking against the catalogue. 0.3375 / 0.013 = 25.96, which is exactly the corneal-power floor Ezer publishes for both the ERK-BH and the ERK-9100. And 0.3375 / 0.0102 = 33.09, matching the 33.00 D floor published by the ERK-H, ERK-9000 A, ERK-9200 and ERK-5400 A alike. The formula and five separate product pages agree, which is a useful thing to know about a set of vendor numbers.

A flatter cornea produces a lower dioptric value, and an instrument whose published range stops at roughly 33 D will not return a figure below it. That is a seven-dioptre difference in published range between the two halves of the line, and it is invisible in any comparison that reports sphere range only — which is nearly all of them.

So the real question is narrower than "which Ezer ARK?" If the wide measurement envelope is what you are buying, you are choosing between exactly two instruments, and they publish identical ranges. What separates them is everything else: the ERK-9100 adds Hartmann-Shack wavefront aberrometry with Zernike mapping on a 6.5 in display, while the ERK-BH puts a larger 7 in tilting and swivelling touchscreen and five named measurement modes — including contact-lens base curve and peripheral keratometry — in front of the same measurement range. Aberrometry, or ergonomics and contact-lens modes. That is the actual decision, and no cross-brand ranking will surface it, because two models from one manufacturer never appear on the same list.

All seven Ezer ERK models, as published

Every figure below is transcribed from each model's own specification table on its product page.

Three things in that table are worth saying out loud, because they cut against the assumptions buyers usually bring to it.

• A smaller screen does not mean a smaller instrument. The ERK-H has the smallest display in the line and occupies exactly the same 260 × 500 × 450 mm as the ERK-BH. If bench space is the constraint, the model to look at is the ERK-5400 A at 248 × 476 mm; if weight is, the ERK-9000 A at 18 kg.
• The largest screen sits on the narrowest measurement range. The ERK-770's 10.4 in panel is the biggest in the line and its sphere range is the smallest. It is bought for automation and connectivity, not for reach.
• One model publishes no corneal-power figure, and one published figure looks wrong. The ERK-770 page publishes no corneal-power range, so the 33.75 D above is derived from its published radius rather than quoted. That page also lists a corneal-astigmatism range of 0.0 to 0.8 D, an order of magnitude narrower than every comparable instrument publishes and almost certainly the same class of decimal slip as the monitor rows above. We have not reproduced it as a specification — confirm it with us before relying on it.

Minimum pupil diameter: the spec that decides whether you get a reading at all

The ERK-BH publishes a minimum pupil diameter of Ø2.0 mm. This figure almost never appears in comparison content, and it is the one that determines whether the instrument returns a number or an error on a small pupil.

It is a competitive figure for the class. In a peer-reviewed evaluation of open-field autorefractors as pupillometers, Otero and colleagues (International Journal of Ophthalmology, 2017;10(4):567–572) record the Grand Seiko WAM-5500's minimum and maximum pupil diameter as 2.00 and 8.00 mm, while the PowerRef II's measurable range runs from 3.00 to 8.00 mm. A 2.0 mm floor puts the ERK-BH at the tighter end of what instruments in this space publish, and a full millimetre below a device that starts at 3.00 mm.

Two honest caveats. Otero's paper is about pupillometry, not about the ERK-BH, and it makes no claim that a small pupil stops an autorefractor from working — it is quoted here only as a published comparator for the number. And the ERK-BH's own 2.0 mm figure is Ezer's published minimum, not an independently verified performance result.

K readings only compare across instruments when the index matches

Ezer qualifies the ERK-BH's corneal-power range carefully: 25.96 to 67.50 D "when cornea equivalent refractive index is 1.3375." That qualifier is doing real work, and it matters on the day you install the instrument next to one you already own.

A keratometer does not measure dioptres. It measures the radius of curvature of the anterior cornea and converts it using an assumed index. Gurnani and Kaur record that "ever since the invention of the keratometer by Helmholtz, the refractive index of the cornea has been 1.3375 for calibrating the instrument," giving the conversion as D = 0.3375 / r. But 1.3375 is a convention, not a universal one. A keratometry primer published by QuickGuide notes that an index of 1.332 is normally used in Europe while 1.3375 is used in the US, and that depending on the device the index "could be as varied as 1.3315, 1.336, 1.3375 and 1.338."

The size of the discrepancy is arithmetic anyone can run. Take a 7.50 mm corneal radius. At 1.3375, D = 0.3375 / 0.0075 = 45.00 D. At 1.332, D = 0.3320 / 0.0075 = 44.27 D. Same eye, same measured radius, a difference of 0.73 D purely from the assumed index. QuickGuide puts the same comparison at up to about 0.70 D.

The practical instruction is short: before you conclude that a new instrument disagrees with the one beside it, check that both are set to the same keratometric index. If you want the underlying mechanics, our explainer on what a keratometer is and how K readings work covers the measurement itself in detail.

What autorefraction settles, and what it does not

An autorefractor keratometer is a starting point for a refraction, not a substitute for one, and the published evidence on the class is worth knowing when you set expectations with a new technician or a new hire.

Kumar and colleagues (PLOS ONE, 2021;16(5):e0251583) compared two autorefractors against subjective refraction in 197 eyes of 104 adults with a mean age of 63. For one instrument the mean difference in spherical equivalent was +0.16 D with 95% limits of agreement from −0.82 to +1.14 D and an intraclass correlation of 0.95; for the other, +0.42 D with limits from −1.30 to +2.14 D and an ICC of 0.84. The authors' own conclusion was that the relatively wide limits of agreement mean subjective refinement of the spectacle prescription is still required.

Kemchoknatee and colleagues (Cureus, 2023;15(4):e37448) ran 48 eyes through two table-top autorefractors and subjective refraction and found the disagreement concentrated in the cylinder rather than the sphere: mean cylindrical differences of −0.91 ± 1.04 D and −0.95 ± 1.06 D, against spherical-equivalent differences of −0.25 and −0.16 D. Their recommendation was that patients with high astigmatism be watched closely when measured by autorefractor, because agreement between objective and subjective refraction is lower there.

Neither study tested the ERK-BH, and no peer-reviewed agreement study for this model was located. These are class findings about what autorefraction does in general, presented separately from the model's published specifications, and the two should not be blended. What they support is a workflow expectation rather than a claim about any instrument: the objective reading is the fast, repeatable starting number, the cylinder is where it most often needs refinement, and the subjective refraction is where the prescription is settled.

Bench, lane and consumables

At 260 × 500 mm of footprint and 20 kg, the ERK-BH is a conventional table-top instrument — but the 500 mm depth is the number to check, because it is the dimension that runs toward the patient and it is what most instrument tables are tightest on. Add the depth of the motorised chinrest travel and the operator's own working room behind the joystick before you assume it drops onto an existing table.

Three more planning items that rarely make a spec comparison:

• The screen is the ergonomic feature. Ezer publishes that the 7 in touchscreen swivels up and down through 180° and left to right through 135–145°. On a shared pretest station used by operators of different heights, that range is worth more day to day than a larger fixed panel.
• The internal printer is a recurring consumable. A thermal line printer with an auto-cutter means thermal roll paper, ordered forever. Ezer publishes an economy print mode that compacts the results to use less of it. Decide at purchase whether you will print at all, or export instead.
• The operating envelope is +10 to +40 °C at 30–85% RH. That is unremarkable for a conditioned exam room and worth a second look for a satellite site, a mobile unit or a room that loses climate control overnight. Storage and transport tolerate −10 to +55 °C.

Getting the data out

The ERK-BH stores ten measured values per eye. That is a working buffer, not an archive — enough for a busy morning, not enough for the instrument to serve as a record system. Ezer publishes USB and RS-232 as the export paths, with the ten most recent exams uploadable at the touch of a button, and describes the instrument as able to connect to other Ezer digital instruments to act as the hub of a digitally enabled exam room.

US Ophthalmic's own product page goes further and lists the ERK-H/BH autorefractor alongside the ELM-BH lensmeter, the EDR-H phoropter and the EDC-BH acuity chart as interacting over Wi-Fi within the Ezer Digital Practice. Note the attribution: that networking description comes from the catalogue listing, while USB and RS-232 are what appear in the manufacturer's own specification table. If a wireless pretest-to-lane data path is part of why you are buying, ask us to confirm in writing which interfaces ship on the unit you are quoted, and which EMR systems have been tested with it. The product page displays partner logos as images with no readable text, so we have not named any EMR platform here.

ERK-BH or the handheld ERH-770?

US Ophthalmic sells both, and the comparison comes up constantly. They are not competing products.

The ERK-BH is a chinrest instrument. It assumes a patient who can sit at a table, and in exchange gives you keratometry, contact-lens base curve, peripheral keratometry, corneal and pupil diameter, an internal printer, a 7 in screen and the widest measurement ranges in the Ezer line. The ERH-770 assumes nothing about posture or position — it goes to the patient — and, as our page on the handheld autorefractor sets out in detail, it publishes no keratometry at all.

That is the whole decision in one line. If any part of your work needs K readings, contact-lens base curve or corneal diameter, a handheld in this class does not replace a table-top ARK — it complements one. Practices that see wheelchair users, paediatric patients, care-home or school screenings typically end up owning both, and use the table-top for everyone who can reach it.

Why buy it from US Ophthalmic

Ezer is our own house brand, which changes the support arrangement in ways that matter more over a ten-year service life than any single specification. There is no chain of intermediaries between you and the people who service the instrument: US Ophthalmic runs an in-house technical and spare-parts department, and the instrument ships with full US warranty and service. When a joystick, a chinrest motor or a printer assembly needs attention years from now, the parts and the technicians sit with the same organisation that sold you the unit. More on how we support what we sell.

You can see the ERK-BH's own listing, images and documentation on the ERK-BH product page, browse the rest of the line in autorefractors and keratometers, and compare it directly against the ERK-9000 A and the ERK-770. For background on where an ARK sits in the exam sequence, see understanding the importance of autorefractors.

Four things to confirm before you sign

1. The keratometric index setting, if the ERK-BH will sit alongside an instrument you already own. Matching indices is the difference between a 45.00 D and a 44.27 D reading on the same 7.50 mm cornea.
2. Which data interfaces ship on your unit, and whether the Wi-Fi path into the Ezer Digital Practice is included or optional — the manufacturer's specification table lists USB and RS-232.
3. Which EMR systems have actually been tested with the export you intend to use. Get the list in writing rather than from a logo strip.
4. Bench depth and operator room. 500 mm of instrument depth plus chinrest travel plus working space behind the joystick is the real footprint, not 500 mm.

Ready to talk specifics? Request a quote on the Ezer ERK-BH and we will confirm current availability, the interfaces on the unit you would receive, and warranty and service terms.

Frequently asked questions

What is the measurement range of the Ezer ERK-BH autorefractor keratometer?

Ezer publishes a sphere range of −30.00 to +25.00 D at a 12 mm vertex distance, cylinder from 0.00 to ±10.00 D, axis 1–180°, and a corneal radius of curvature from 5.0 to 13.0 mm, which corresponds to 25.96 to 67.50 D of corneal power at an index of 1.3375. Sphere and cylinder are selectable in 0.12 or 0.25 D increments.

What is the smallest pupil the ERK-BH can measure through?

Ezer publishes a minimum pupil diameter of 2.0 mm. For context, a 2017 peer-reviewed evaluation records the Grand Seiko WAM-5500 at a 2.00 mm minimum and the PowerRef II at 3.00 mm, so a 2.0 mm floor is at the tighter end of what instruments in this space publish.

How is the ERK-BH different from the other Ezer ERK models?

On measurement range it shares the top of the line with the ERK-9100 rather than leading alone: both publish −30.00 to +25.00 D of sphere and a corneal radius of 5.0 to 13.0 mm, against −25.00 to +22.00 D and 5.0 to 10.2 mm on the ERK-H, ERK-9000 A, ERK-9200 and ERK-5400 A, and −20 to +20 D with a 10.0 mm radius limit on the ERK-770. What separates the ERK-BH from the ERK-9100 is not range but everything else: the ERK-9100 adds wavefront aberrometry on a 6.5 in display, while the ERK-BH offers a larger 7 in tilting and swivelling touchscreen and five measurement modes including contact-lens base curve and peripheral keratometry.

Does the ERK-BH connect to an EMR?

The manufacturer's specification table lists USB and RS-232 as the export interfaces, and the US Ophthalmic listing describes the ERK-H/BH working over Wi-Fi with the ELM-BH lensmeter, EDR-H phoropter and EDC-BH acuity chart inside the Ezer Digital Practice. Because those are two different sources, confirm with us in writing which interfaces are on the specific unit you are quoted and which EMR platforms have been tested with it.

Can an autorefractor replace a subjective refraction?

The published evidence on the class says no. In a 2021 PLOS ONE study of 197 adult eyes, spherical-equivalent limits of agreement against subjective refraction ran from −0.82 to +1.14 D for the better-performing instrument, and the authors concluded subjective refinement of the prescription was still required. A 2023 Cureus study of 48 eyes found the disagreement concentrated in the cylinder. Neither study tested the ERK-BH; both describe the class.

How much space does the ERK-BH need?

The published dimensions are 260 mm wide, 500 mm deep and 450 mm high, at roughly 20 kg. The 500 mm depth is the constraint on most instrument tables, and you should add motorised chinrest travel and operator working room behind the joystick before assuming it fits an existing station.

Does the ERK-BH measure corneal diameter and contact lens base curve?

Yes. Ezer publishes a CLBC mode for contact-lens base curve, a corneal-diameter range of 2.0 to 14.0 mm in 0.1 mm increments, and a freeze mode in which corneal, pupil and hard contact-lens diameters are measured by touching and dragging on the touchscreen.