8.1 Lensometry Overview
Key Takeaways
- A lensometer (lensmeter, vertometer, focimeter) measures the back vertex power of a spectacle lens: sphere, cylinder, axis, add, prism, and optical center.
- The Certified Ophthalmic Assistant (COA) exam has 200 multiple-choice questions, a 180-minute limit, and is delivered at Pearson VUE; scoring is criterion-referenced (modified Angoff) with no published passing percentage.
- Two manual mire styles exist: the American cross (three thin sphere lines plus a single thick cylinder line) and the European dot pattern.
- Plus-cylinder and minus-cylinder readings describe the same lens; the COA convention is usually minus-cylinder transposition for the clinic record.
8.1 Lensometry Overview
Lensometry is the measurement of the optical properties of a finished spectacle lens. The instrument is called a lensometer (also lensmeter, vertometer, or focimeter). It reads the back vertex power of the lens, meaning the power referenced to the back surface that sits closest to the eye. A COA performs lensometry to verify that a new pair of glasses matches the written prescription, to read an unknown old pair so the doctor has a baseline, and to confirm segment height, optical center, and any prescribed prism.
What the lensometer reports
A complete lensometry reading has up to six parts, and the exam expects you to name all of them:
| Element | What it tells you | Units |
|---|---|---|
| Sphere | Overall spherical power (myopic = minus, hyperopic = plus) | Diopters (D) |
| Cylinder | Power difference between the two principal meridians (astigmatism) | Diopters (D) |
| Axis | Orientation of the cylinder, 1 to 180 degrees | degrees |
| Add | Extra plus power in the near segment of a bifocal/progressive | Diopters (D) |
| Prism | Light-bending power and base direction at the optical center | prism diopters (Δ) |
| Optical center / PD | The no-prism point, marked to confirm fit | mm |
Exam logistics that anchor this domain
The COA examination is administered by IJCAHPO (International Joint Commission on Allied Health Personnel in Ophthalmology). It is a 200-question multiple-choice test with a 180-minute time limit, delivered at Pearson VUE centers or by OnVUE remote proctoring. Scoring is scaled, and the practical passing standard works out to roughly 70-75% correct (about a scaled 72). Lensometry questions sit inside the broader optics and refractive-procedures content and are tested as hands-on tasks, not trivia.
Two mire styles
Manual lensometers display a mire target you bring into focus. The American cross uses three thin lines (the sphere lines) crossing a single thick line set (the cylinder lines). The European dot style uses a ring of dots that elongate into a line when the cylinder meridian is in focus. Automated lensometers print sphere, cylinder, axis, add, and prism digitally, but the COA may be expected to confirm a reading manually.
Sign-of-cylinder convention
The same lens can be written in plus-cylinder or minus-cylinder form; the two are mathematically identical. Ophthalmology clinics and the COA record most often use minus-cylinder, while many opticians grind in plus-cylinder. You convert between them by transposition (covered in 8.2). Always note which convention a stem is using before you compare a reading to the chart, because -2.00 +1.00 x 090 and -1.00 -1.00 x 180 describe the very same lens.
Why neutralization matters
Reading a lens is really neutralization: you spin the power drum until the mire lines snap into sharp focus, which means the lens power has been cancelled by the instrument's optics. A blurry or doubled mire means you have not reached neutral. The most common beginner error is reading the front vertex power; always seat the lens with the back surface against the lens stop.
Diopters, the working unit
Every power the lensometer reports is in diopters (D), defined as the reciprocal of the focal length in meters: a 1.00 D lens focuses parallel light at 1 meter, a 2.00 D lens at 0.5 meter. Plus (convex) lenses converge light and correct hyperopia (farsightedness) and presbyopia; minus (concave) lenses diverge light and correct myopia (nearsightedness). Spectacle lenses are made in 0.25 D steps, which is why a reading of +2.10 D is unusual and signals you should recheck calibration.
Cylinder corrects astigmatism, where the cornea or lens is more steeply curved in one meridian than the other, so the eye has two focal points rather than one. The cylinder's axis tells you the meridian that carries no cylinder power.
Where lensometry fits in the clinic
The COA's lensometry duties are practical and recurring. You verify a finished pair against the doctor's written spectacle prescription (Rx) before dispensing, you neutralize a patient's current glasses at check-in to give the physician a starting point for refraction, and you flag any mismatch that could explain a complaint of blur, eyestrain, headache, or double vision. You also confirm the segment height on bifocals and progressives and verify any prescribed prism, which the doctor orders to relieve double vision in conditions such as a phoria or tropia.
Recording the optical center and interpupillary distance (PD) confirms the lens is positioned so the patient looks through the no-prism point rather than through an off-center zone that would induce unwanted prism.
Reading the lens record correctly
A spectacle Rx is written as sphere, cylinder x axis, with add and prism appended when present, and always separated into OD (oculus dexter, right eye) and OS (oculus sinister, left eye); both together are OU (oculus uterque). When you compare a lensometer reading to the chart, match eye to eye and confirm the cylinder convention before judging any difference. A reading written without an axis (for example, +1.50 with no cylinder) is a spherical lens, and its mire lines all focus at the same drum position.
On a lensometer, what does the term "back vertex power" describe?
Which combination correctly lists the elements a complete lensometry reading can include?