A lensmeter turns patterns of light into a prescription. Once you understand the two principal meridians, the reading becomes much less mysterious.
What does a lensmeter measure?
A lensmeter (also called a lensometer or focimeter) measures the power of an existing spectacle lens. Depending on the instrument and technique, you can also find cylinder axis, optical-center position and prism. In practice, it helps verify that a finished pair of eyeglasses matches its intended prescription.
Before you measure: set up the instrument
- Focus the eyepiece reticle for your own eye, using the manufacturer's procedure, so accommodation does not shift your readings.
- Check the zero reading with no lens at the stop.
- Place the spectacle lens correctly against the lens stop and keep the frame stable on the table.
- Measure the right lens (OD) and then the left lens (OS), recording both separately.
How do two focused positions give cylinder power?
A spherical lens has the same power in all meridians, so the line targets focus together. A sphero-cylindrical lens has two principal meridians, at right angles to one another. The power drum brings those meridians into focus at two different settings.
| Observation | Lensmeter reading |
|---|---|
| First (more plus) principal-meridian reading | +2.00 D |
| Second principal-meridian reading | +1.00 D |
| Axis aligned for first principal meridian | 148° |
Cylinder = second power − first power = +1.00 − (+2.00) = −1.00 D.
The perpendicular meridian is 58°. The axis tells you the meridian with zero additional cylinder power, not the meridian of maximum cylinder effect.
A repeatable lensmeter workflow
- Find and align the first focused target using the instrument's axis control.
- Record the first power and axis without accidentally changing the spectacle position.
- Focus the perpendicular target, and record the second power.
- Subtract the first reading from the second to obtain cylinder in the intended notation.
- Repeat for the other lens. Check for any prescribed prism or addition using the proper reference points and method.
In the LearnOptix Lensmeter Simulator, try working through both OD and OS rather than treating each lens as an unrelated prescription.
Common mistakes to avoid
- Skipping eyepiece calibration: the power drum may be read inaccurately if your eye accommodates.
- Mixing cylinder conventions: check whether you are measuring or transposing in plus or minus cylinder.
- Confusing axis with the other principal meridian: axis orientation and cylinder effect are not interchangeable.
- Ignoring lens position: incorrect seating or measuring away from the designated point may change the reading.
Try this
You find the first meridian at −1.50 D, the second at −2.25 D, and the first-meridian axis at 173°. What prescription would you record in minus-cylinder format?
Show answer
−1.50 D Sph −0.75 D Cyl x 173. Subtract −2.25 − (−1.50) = −0.75 D.
References and further reading
- EyeWiki (American Academy of Ophthalmology). “Lensometry.” Overview of lensmeter functions and use.
- Vision Expo. “Introduction to Lensometry” (course handout). Manual lensmeter setup and neutralization steps.