Lensometer simulator

How a lensometer works, in plain words

A lensometer (also called a lensmeter or focimeter) measures the power of a lens. Light from a lit target passes through the lens into a small telescope. The power wheel moves the target until it looks sharp; at that point the wheel reading equals the lens power in that direction. A lens with astigmatism (a cylinder) has two different powers 90° apart, so you get two readings: the thin sphere lines focus at one, the thick cylinder lines at the other.

Steps

1. Focus the eyepiece. With no lens in place, turn the eyepiece out until the reticle rings blur, then turn it back slowly and stop as soon as they are sharp. The target should then focus at 0.00 on the power wheel.
2. Center the lens. Set the glasses on the lens table, back surface against the lens stop, and slide them until the target sits in the center of the reticle.
3. Find the sphere lines. Turn the power wheel and the axis wheel together until the thin sphere lines are sharp and unbroken. For minus-cylinder form, start from the plus end so the first lines you reach are at the most plus power.
4. Read the sphere and axis. The power wheel now shows the sphere. The axis wheel shows the axis.
5. Read the cylinder. Keep turning the power wheel (do not touch the axis wheel) until the thick cylinder lines are sharp. Cylinder = second reading minus first reading.
6. Spot the optical center and prism. With the target centered, use the marking pins to dot the optical center. If you cannot center the target where the pupil should be, count the rings to the target center: each ring is 1 prism diopter, and the base is the side the target moved toward.

Example (minus cylinder): sphere lines sharp at −2.00 with the axis wheel on 180, cylinder lines sharp at −3.00. Cylinder = −3.00 − (−2.00) = −1.00. Reading: −2.00 −1.00 × 180, which is −3.00 +1.00 × 090 in plus-cylinder form. Practice the conversion on the transposition calculator.

Plus-cylinder form: do the same, but focus the sphere lines at the least plus (most minus) reading first, then turn toward plus for the cylinder lines.

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Reading the add on bifocals and progressives

Read the distance power first. Then move the lens so the near zone (the bifocal segment, or the bottom of a progressive's corridor) is over the lens stop and refocus the same lines. Add = near reading minus distance reading. Adds are made and measured as front vertex power, so turn the glasses around (front surface against the lens stop) for both the distance and near readings when you measure an add. Distance power is read as back vertex power, with the back surface against the stop.

Common mistakes

• Skipping the eyepiece focus, or focusing it by turning back and forth. An unfocused eyepiece shifts every reading.
• Reading the second power as the cylinder. Cylinder is the difference between the two readings.
• Touching the axis wheel while reading the cylinder.
• Mixing up forms: minus cylinder starts at the most plus reading, plus cylinder at the most minus.
• Reading a strongly decentered lens away from the optical center, which adds prism and can make the lines hard to center.
• Measuring an add with the back surface against the stop instead of the front.

Frequently asked questions

What is the difference between a lensometer, lensmeter and focimeter?

There is no difference: they are three names for the same instrument, which measures the power of eyeglass lenses. Lensometer and lensmeter are the usual US terms; focimeter is the term used in international standards and in many other countries. You may also hear vertometer.

How do you read a lensometer?

Focus the eyepiece, center the lens against the lens stop, then turn the power and axis wheels until the thin sphere lines are sharp and read the sphere and axis. Next turn only the power wheel until the thick cylinder lines are sharp; the cylinder is the second reading minus the first.

How do you find the axis on a lensometer?

Turn the axis wheel while you focus until the thin sphere lines are sharp, straight and unbroken; the axis wheel then shows the axis. Do not touch the axis wheel again while you read the cylinder.

What does each ring on the reticle mean?

On a standard manual lensometer each ring is 1 prism diopter (1Δ) from the center. Count rings from the center to the target center.

How do you measure prism on a lensometer?

Mark the point the wearer looks through (the pupil position, or the prism reference point on a progressive), put that point at the center of the lens stop, and count the rings from the reticle center to the target center. For more prism than the rings show, use the instrument's prism compensator or hold loose prisms over the lens.

See also: Prentice's rule calculator

How do I know the base direction?

The base is the direction the target moved: toward the nose is base in, toward the temple base out, up is base up, down is base down. Some instruments flip the image, so check yours.

How do you read the add on a bifocal or progressive?

Add = near reading minus distance reading. Turn the glasses so the front surface is against the lens stop, read the distance zone, then the bifocal segment or the bottom of the progressive corridor, and subtract.

What if the sphere and cylinder lines focus together?

The lens is spherical (no cylinder). Record the sphere and write DS or "sph".

Is this the same as an automated lensmeter?

No. Automated lensmeters do the focusing for you. Learning the manual instrument helps you understand what the numbers mean and is part of many optician skills checks.

See also: ABO certification course

Preparing for the optician exam? See the ABO certification course.

Also try the keratometer simulator and the Prentice's rule calculator.