Lensometry for the COA: Sphere, Cyl, Axis, Add, Prism

Lensometry is 3% of the COA® exam in IJCAHPO’s content areas effective August 1, 2025. Set the eyepiece first, center the lens, and clear the single lines for sphere. Turn to the triple lines for cylinder and read the axis wheel. Find the add as the difference between distance and segment readings, then read prism from the rings.

Last checked Nov 2, 2026. Confirm fees and dates with the certifying body. optical.education is independent and is not affiliated with or endorsed by IJCAHPO. Disclaimer.

How much of the COA exam it is

IJCAHPO’s COA content areas effective 8/1/2025 give lensometry 3% of the exam (checked Nov 2, 2026). The Core Criteria Handbook lists the COA tasks as neutralizing spectacles, on both automated and manual instruments. Fresnel, ground-in and slab-off prism appear under COT and COMT, so this page treats prism as a reading skill only. The handbook also prints an older percentage table; we use the 2025 PDF.

Eyepiece focus

Your own eye can add error before a lens is even mounted. With nothing on the stage:

Turn the eyepiece fully out, toward plus.
Look at the reticle and turn the eyepiece back slowly.
Stop the moment the reticle is sharp. Turning past it adds minus that your accommodation will hide.

Sphere, cylinder and axis

Seat the glasses with the bottom of the frame toward you and the frame table against them, then line the target up with the reticle center. Read the right lens, then slide across to the left lens with the table height unchanged, so vertical prism is not missed.

Sphere: turn the power wheel until the single (narrow) lines are crisp and unbroken. If the lines look broken, rotate the axis wheel until they join. For a spherical lens, every line clears at once.
Cylinder: keep turning until the triple (wide) lines clear. The cylinder is the step between the two readings, with its sign set by the direction you turned.
Axis: read it from the axis wheel at the single-line setting.

Turning toward minus for the second lines gives minus cylinder; turning toward plus gives plus cylinder. Either form is the same lens. See plus and minus cylinder for converting between them.

Try the simulator

The lensometer simulator lets you turn the power and axis wheels on a virtual lens. Set a lens, read it the way described above, and check your answer.

Measuring the add

A standard lensometer reads back-vertex power. For the add, textbooks have you turn the glasses around so the front surface faces the lens stop. Read the distance portion just above the segment, then raise the frame table to read through the segment without moving the frame by hand.

The add is the difference between the two readings. Use the same lines both times: single lines with single lines, or triple with triple. Each step on the power wheel is 0.25 D, so you can also count the steps.

Reading prism

With the lens centered at the measuring point, a displaced target means prism. Each ring of the reticle marks 1Δ. The base points the way the target moved: toward the nose is base in, toward the temple base out, up base up, down base down. Some reticles carry the scale on lines instead of rings; the logic is the same.

Recording

A common convention is to write each eye on its own line, OD first, then OS. Give sphere and cylinder with their signs and two decimal places (+0.75, not +.75), and the axis with no degree sign, often as three digits (015, not 15), so it cannot be misread. Follow with the add and any prism with its base. Say whether you read plus or minus cylinder if your clinic records both. Brooks and Borish describe the decimal, degree-sign and three-digit-axis habits; Ledford’s answer key uses the same layout.

Practice set

Original problems. Work each before opening the answer.

The single lines clear at −0.75 with the axis wheel at 15. Turning further minus, the triple lines clear at −2.25. Write the lens.
Show answer
Cylinder = −2.25 − (−0.75) = −1.50. −0.75 −1.50 × 015
The single lines clear at +1.50 with the wheel at 140. The triple lines clear at +3.25 when you turn toward plus. Write the lens in plus cylinder.
Show answer
Cylinder = +3.25 − (+1.50) = +1.75. +1.50 +1.75 × 140
Glasses turned around (front vertex). Single lines clear at +0.50 through the distance part and at +2.75 through the segment. What is the add?
Show answer
+2.75 − (+0.50) = +2.25 add
Single lines clear at −1.50 in the distance part and at +0.75 in the segment. What is the add?
Show answer
+0.75 − (−1.50) = +2.25 add. Count it on the wheel: 9 quarter-diopter steps.
Left lens. The target center sits on the second ring, displaced straight up. How much prism, which base?
Show answer
Each ring is 1Δ and the base lies the way the target moved: 2Δ base up.

Standard texts used for this page: Ledford JK. Certified Ophthalmic Assistant Exam Review Manual, 3rd ed. SLACK, ch. 5 (lensometry answers); Brooks CW, Borish IM. System for Ophthalmic Dispensing, 3rd ed. Butterworth-Heinemann, ch. 6 (lensmeter use, add by front vertex, prism); and the University of Iowa Optics Review, linked under sources.

COA® is a registered mark of IJCAHPO.

Frequently asked questions

How much of the COA exam is lensometry?

Lensometry is 3% of the COA exam in IJCAHPO’s content areas effective August 1, 2025. The handbook lists the COA tasks as neutralizing spectacles on manual and automated instruments.

Sources: IJCAHPO — COA Examination Content Areas, effective 8/1/2025 (PDF) (opens in a new tab) ; IJCAHPO — Core Certification Criteria Handbook (PDF) (opens in a new tab)

How do you focus a lensometer eyepiece?

Before any lens goes on, turn the eyepiece out to its most plus setting, then turn it back slowly and stop as soon as the target looks sharp. Going past that point puts your own focusing into the reading. Do it again whenever someone else has used the instrument.

Official sources

Last reviewed Nov 2, 2026. Requirements, fees, and dates change — always confirm with the official source before you register or apply. ABO-NCLE and IJCAHPO are the only authorities for their own exam facts, so those facts rest on a single source.