Skill builder: Manual lensometry

By Brenea Facchini, BS, COT, CPC, OSC

Manual lensometry is the gold standard in neutralizing eyeglasses. Understanding how to use the manual lensometer properly guarantees the correct starting point for refractions and provides an accurate neutralization.

Ensure that you are using the manual lensometer accurately with the following guide:

1. Turn the lensometer on and focus the eyepiece. Set the power drum to zero and spin the reticle (eyepiece) until all lines are into focus.
2. Place the spectacles on the stage with temples facing away from you and center the part of the eyeglasses you intend to read on the lens stop. Lock the spectacles into place.
3. Get spherical lines (narrow lines) into focus first by turning the power drum. The lines may be broken if there is astigmatism — if so, spin the cylinder axis wheel to straighten out the spherical lines.
4. If the narrow lines and wider lines both come into focus at the same time while turning the power drum, this indicates there is no astigmatism correction. If the glasses are single vision, the neutralization is complete. If the glasses are bifocals, you’ve completed reading the distance portion of the glasses. If the narrow lines and wider lines do not come into focus at the same time, we can identify the amount of astigmatism in the glasses after getting the spherical lines into focus. If there is astigmatism correction, determine what cylinder power you will need to measure the astigmatism.

For minus cylinder, follow these steps:

1. Set the power drum to high-minus sphere.
2. Turn the power drum away from you until the narrow lines come into focus. If the cylinder mires come into focus first, rotate the axis wheel by 90°.
3. Read the power drum. This is the spherical power.
4. Continue turning the power drum away from you until the wider lines come into focus and read the power. If they do not focus, turn the axis wheel until they appear clear.
5. For cylinder power, calculate the difference between the first measurement (from Step 3) and the second reading (from Step 4). For example: sphere reads +2.25, and cylinder reads +0.75; the cylinder power is -1.50.

For plus cylinder, follow these steps:

1. Set the power drum to high-plus sphere.
2. Turn the power drum towards you until the narrow lines come into focus. If the cylinder mires come into focus first, rotate the axis wheel by 90°.
3. Read the power drum. This is the spherical power.
4. Continue turning the power drum towards you until the wider lines (cylinder) come into focus and read the power. If they do not focus, turn the axis wheel until they appear clear.
5. Calculate the difference between the first measurement (from Step 3) and the second reading (from Step 4). This is the cylinder power. For example: sphere reads +2.25, cylinder reads +2.75; cylinder power is +0.50.

To neutralize the bifocal segment:

1. Lift the spectacles and center the bifocal segment on the lens stop. Lock into place.
2. Rotate the power drum towards you until the sphere lines are in focus again and read the power.
3. Calculate the difference between this reading and the distance spherical reading. This is the add power, which is always expressed in plus.

Tips for successful readings

Common errors to avoid include:

• Not focusing the eyepiece before placing the lenses on the stand. If the eyepiece is out of focus, you may find your readings off by half of a diopter or more.
• Confusing plus with minus cylinder
• Neutralizing the add power on the incorrect segment of the lens.

In addition, be careful to neutralize the lenses in the correct cylindrical power — otherwise the refractionist will need to transpose before starting their refraction. Also, when reading a lined trifocal, double-check to make sure the larger reading portion of the lens is centered on the lens stop, not the smaller trifocal segment. OP