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Lensometry is the process of measuring the refractive power of existing ophthalmic lenses using a lensometer (also called a focimeter or vertometer). It is one of the most commonly performed tasks by ophthalmic assistants in every ophthalmology and optometry practice. Before a refraction begins, the patient's existing glasses are lensometered to document the habitual correction — the prescription the patient has been using. This baseline measurement is essential for comparing with the new refraction and for identifying any discrepancies between what was prescribed and what was dispensed.
On the COA exam, lensometry falls within the Assessments domain (42% of the exam) as part of the lensometry sub-topic. You are expected to understand the components being measured, the technique for both manual and automated instruments, and how to interpret and record findings including sphere, cylinder, axis, prism, and add power.
The lensometer measures the back vertex power of a lens — the vergence power at the back surface of the lens as it sits in its final position in the spectacle frame. For most clinical purposes, this is the relevant power, because it corresponds to the vergence of light reaching the eye. The instrument contains a calibrated lens system, a moveable mire (the target pattern), and an eyepiece through which the examiner views the mire through the test lens.
Sphere Power
Measured in diopters (D), positive or negative
Cylinder Power & Axis
Measured in diopters and degrees (1-180°)
Prism & Add Power
Prism in diopters; Add in diopters
The manual lensometer requires the examiner to manipulate controls while viewing the mire through the eyepiece. The mire pattern consists of a set of lines (typically three parallel lines intersecting another set of three lines at 90 degrees, forming a cross pattern). Reading sphere, cylinder, and axis from a manual instrument requires a specific sequence of steps that the COA exam tests.
Before testing any lenses, adjust the eyepiece to correct for your own refractive error. With no lens in the instrument, rotate the eyepiece until the mire lines are sharpest. This compensates for the examiner's refractive error and prevents it from influencing the reading. This step is essential and is frequently missed by beginners — the COA exam tests knowledge of this preliminary step.
Hold the frame temples pointing away from you (as worn by the patient). Place the right lens (OD) against the lens stop first. The glasses are positioned as if looking through the back of the lens (the lens back surface faces the examiner). This orientation gives back vertex power, which is what is prescribed and dispensed.
Rotate the axis wheel until one set of three lines (the sphere lines) comes into sharp focus while the other set remains blurred. Read the sphere power from the diopter wheel at the position where those three lines are sharpest. The sphere is always the power at which the first set of lines clears (using minus-cylinder format, the most common in US practice).
Without moving the axis wheel, rotate the diopter wheel in the minus direction until the second set of three lines also comes into focus. The cylinder power is the difference between the two wheel readings (the amount you had to move the wheel in step 4). Read the axis from the axis scale — this is the orientation at which you were holding the axis wheel when the first lines cleared in step 3.
Move the glasses frame until the mire cross-hairs are centered with no ring displacement — this is the optical center. Use the marking device to dot the lens. If the mire is displaced from center when the cross-hairs are centered on the dot, prismatic effect is present. The amount of prism can be read from the scale, and the base direction is determined by the direction of mire displacement.
The appearance of the mire in the lensometer eyepiece tells you about the lens power and orientation. Understanding what different mire appearances mean is essential for troubleshooting and confirms your readings. The COA exam may present mire descriptions and ask you to identify what they indicate.
| Mire Appearance | What It Indicates |
|---|---|
| Both sets of lines sharp and centered | No cylinder (spherical lens) or axis aligned correctly; sphere power is correct at this diopter wheel setting |
| One set of lines sharp, other blurred | Cylinder is present; the blurred lines will sharpen when the diopter wheel is advanced; the difference = cylinder power |
| Lines clear but shifted from center | Prismatic effect present; the amount of shift indicates prism magnitude; direction of shift indicates base orientation |
| Lines at 45 degrees (oblique) | Cylinder axis is oblique; the axis wheel must be rotated to the oblique meridian before the lines can be sharpened independently |
| All lines remain blurred regardless of wheel position | Lens is extremely powerful, patient has a dirty or scratched lens, or axis wheel is far from correct orientation; recheck alignment |
Bifocal and progressive lenses have two optical zones: the distance zone (what is recorded as the main prescription) and the near zone (the add power). To read add power, you must measure the distance zone and the near zone separately, then subtract to find the add.
Step 1: Read the distance zone
Position the distance portion of the lens (upper segment) in the instrument. Neutralize and record sphere, cylinder, and axis exactly as with any lens.
Step 2: Read the near zone
Move the glasses frame so the near segment (bifocal segment or the near zone of the progressive) is positioned in the instrument. Record the sphere power in the near zone. The cylinder and axis are usually the same as the distance zone.
Step 3: Calculate the add
Add power = Near sphere power minus Distance sphere power. For example, if distance sphere is -2.50 and near sphere is -0.50, the add power is +2.00 D. Add power is always positive (it adds plus power to the distance prescription for near focus).
Opterio covers lensometry technique, prescription reading, and all other COA assessment topics with adaptive practice questions and AI explanations.
Modern automated lensometers (auto-lensometers) use electronic sensors to detect the position of the mire and calculate sphere, cylinder, axis, and prism automatically. Most models also have a dedicated near-zone reading mode for bifocal and progressive lenses. The examiner positions the glasses in the instrument, places the lens against the sensor, and presses a button — the display shows the prescription values in seconds. Many models print results directly.
Despite the convenience of automated instruments, the COA exam still tests knowledge of manual lensometer technique because manual instruments remain in use in many settings and the procedural knowledge demonstrates understanding of the underlying optics. Understanding how a manual instrument works also helps you troubleshoot automated instruments when they produce unexpected readings.
Lensometry findings must be documented in a consistent, standardized format. The COA exam may test your ability to correctly record findings or identify errors in recorded values.
OD (Habitual): -3.25 -1.75 x 165 Add: +2.25
OS (Habitual): -2.50 -0.75 x 010 Add: +2.25
Full format, content domains, eligibility, and registration details.
Understanding prescriptions, sphere, cylinder, axis, and autorefraction.
Complete VA testing technique and documentation standards.
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The lensometer (also called a focimeter or vertometer) measures the refractive power of ophthalmic lenses in existing glasses. It determines the sphere power, cylinder power, cylinder axis, prismatic power and base direction, and the add power of bifocal or progressive lenses. The lensometer reading gives the prescription that is ground into the lens, which may differ from the patient's current refraction if the prescription has changed.
Reading the patient's existing glasses (called "neutralizing" the glasses or taking "habitual correction") is a standard first step in a comprehensive exam. It provides the baseline correction the patient has been wearing, allows comparison to the new refraction to identify prescription changes, establishes a starting point for the refraction (especially via autorefractor in conjunction), and documents the prescription for the medical record. It also catches dispensing errors — occasions where what was dispensed differs from what was prescribed.
Hold the frame with the temples pointing away from you (as if the patient were wearing them) and place the right lens against the lens stop first — this follows the convention of testing OD first. For glasses with progressive lenses, tilt the frame so the reading segment is forward (downward in the instrument's orientation) to locate the distance and near optical centers sequentially. Always rest the lens gently against the lens stop; pressing too hard can warp flexible frames and give inaccurate readings.
A manual lensometer (such as the American Optical or Reichert models) requires the examiner to rotate the eyepiece and lens wheel to bring the mire into focus and align the cross-hairs, then read the values from the wheel and scale. An automated lensometer (auto-lensometer or lensmeter) projects a target through the lens and electronically calculates sphere, cylinder, axis, and sometimes prism automatically. The printout can be attached to the chart. Automated versions are faster and reduce examiner variability, but manual instruments are still used and are commonly tested on the COA exam.
With the lens in the lensometer, move the glasses until the mire (the test target pattern inside the instrument) is centered in the cross-hairs with no prismatic displacement. This is the optical center — the point where the lens exerts no prismatic effect. Mark the optical center on the lens with a felt-tip marker or the built-in marking device (a small ink stamp). The PD (pupillary distance) and optical center placement affect whether the patient experiences unwanted prism during wear.