How to Use a Lensometer, Manual and Automatic

What a lensometer measures

A lensometer (also called a lensmeter or focimeter) measures the back vertex power of a spectacle lens: sphere, cylinder and axis. It also reads the add of a multifocal and any prism, and marks the optical center with ink dots.

In an optical shop it is used three times on a typical job: to verify lenses when they arrive, to find and mark the optical center before the lens is blocked for edging, and to check the finished glasses against the prescription before they go to the customer.

Before you start: focus the eyepiece

On a manual lensometer, every reading depends on this step. Each operator should do it for their own eye.

1. Set the power wheel to zero.
2. Turn the eyepiece fully counter-clockwise, so the reticle is blurred.
3. Turn it slowly clockwise until the reticle lines are just sharp, and stop there.
4. Check that the target (the mires) is in focus at 0.00 with no lens in place. If it is not, note the difference and correct every reading by that amount.

Position the glasses

1. Clean the lenses; dust and fingerprints blur the target.
2. Put the glasses on the lens table with the back surface against the lens stop and the temples pointing away from you.
3. Make sure both lenses rest flat on the table. A tilted frame reads false cylinder.
4. Move the glasses until the target is centered in the reticle, then lock the lens holder.

Read sphere, cylinder and axis

The common target is a single line for sphere, crossed by a set of three lines for cylinder. The steps below use the minus-cylinder method.

1. Turn the power wheel from the plus side until the single line comes into focus. If it looks broken or tilted, turn the axis wheel until it is straight and continuous.
2. Record the power on the wheel as the sphere.
3. Keep turning in the same direction until the set of three lines comes into focus.
4. The difference between the two readings is the cylinder. With the minus-cylinder method it is a minus value.
5. Read the axis from the axis wheel and write it as three digits: 090, not 90.
6. If both line sets focus at the same power, the lens is spherical and there is no cylinder.

Approach the reading from one direction and do not rock the wheel back and forth; that keeps your own focusing (accommodation) from shifting the result. Sources differ on whether to start from high plus or from a high minus value, but they all agree on not oscillating.

To convert to plus-cylinder form, add the cylinder to the sphere to get the new sphere, change the sign of the cylinder, and change the axis by 90°. For example, −2.00 −1.00 × 180 becomes −3.00 +1.00 × 090.

Read the add of bifocals and progressives

• Bifocals: measure the distance power first. Then move the glasses so the segment is centered, refocus the lines, and subtract the distance reading from the near reading. The difference is the add.
• Progressives: read the distance power through the distance circle near the top of the lens, not at the geometric center. Use the engraved marks and the manufacturer’s layout card to find the add; reading the add through the corridor on a manual instrument is unreliable.
• On an automatic lensmeter, follow its progressive mode, which guides you from the distance zone down to the near zone.

Read prism

Measure prism at the marked pupil position, not at the optical center. The reticle has rings: on many instruments the first ring is 0.5 prism diopters and each further ring 1 prism diopter.

The target moves toward the base. If it sits above center, the prism is base up; if it sits toward the nose, base in. Read the amount from the ring it reaches. Automatic lensmeters show prism in x-y form (horizontal and vertical components) or in polar form (amount and base direction).

Mark the optical center

Center the target in the reticle, check that the lens has not moved, and press the marking device. The middle dot marks the optical center, and the three dots together mark the 180° line. On coated lenses, marks show more clearly with ink made for them.

Using an automatic lensmeter

1. Check the reading on a known lens at the start of the day.
2. Clean the lens and seat it on the nosepiece, back surface down.
3. Let the instrument detect the lens type, or select single vision, bifocal or progressive.
4. Center the lens using the on-screen guide and let the reading settle.
5. Save or print the reading, and mark the optical center if you are going to block the lens.
6. For the second lens, move the frame across without lifting it, so the PD reading stays consistent.

An automatic instrument removes most operator variation, but it cannot correct a dirty or tilted lens. Keep the nosepiece clean and the frame flat.

Common errors and how to avoid them

• Dirty or scratched lenses: clean first.
• A tilted frame: keep both lenses on the table; tilt creates false cylinder.
• Missing low cylinders around 0.25 D: turn slowly and look carefully at the three lines.
• An eyepiece that was never focused, or an uncorrected zero error.
• Reading a progressive at the wrong point: use the distance circle and the engravings.

Checking against tolerances

In the United States, finished glasses are checked against ANSI Z80.1. The 2015 quick reference from The Vision Council lists, among others:

A newer edition of the standard exists, and other countries use their own standards, so check the current values that apply to you.

Care of the instrument

• Check the zero at the start of each session.
• Keep the lens stop, nosepiece and eyepiece clean.
• Replace marking ink, and printer paper on automatic models, before they run out.