Lensmeter: Measuring Sphere Power in Spectacle Lenses

Measuring Sphere Power on the Lensmeter

Measuring sphere power is the most fundamental lensmeter skill. A spherical lens has the same power in all meridians, meaning it refracts light uniformly regardless of orientation. When a purely spherical lens is placed on the lensmeter, both sets of target lines (mires) come into sharp focus simultaneously at a single power drum setting.

Step-by-Step Procedure

1. Focus the eyepiece: Adjust the eyepiece until the reticle (crosshair) is perfectly sharp. This compensates for your own refractive error.
2. Place the lens: Set the spectacle lens on the lens stop with the back (concave) surface facing you. The lens should rest flat and stable against the stop.
3. Rotate the power drum: Turn the power drum slowly while observing the target through the eyepiece.
4. Focus the target: Continue turning until both sets of target lines (thin and thick) appear sharp and clear at the same time.
5. Read the power: Note the reading on the power drum. This is the sphere power in diopters.

Understanding the Target Lines

The lensmeter target consists of two sets of perpendicular lines:

• Set 1 (thin, continuous lines): Represents one principal meridian
• Set 2 (thick, broken/dashed lines): Represents the perpendicular meridian

For a sphere-only lens, rotating the axis wheel has no effect because there is no cylinder component. The lens has identical power in every direction, so the orientation does not matter. Both line sets focus simultaneously regardless of axis wheel position.

Plus vs. Minus Sphere Power

The power drum is marked with plus (+) and minus (-) values:

• Plus readings: The lens converges light (convex lens), used for farsightedness (hyperopia) correction
• Minus readings: The lens diverges light (concave lens), used for nearsightedness (myopia) correction
• Zero (plano): No optical power, the target focuses when the drum reads 0.00

Centering the Target

When the target lines are focused, they should be centered on the reticle (crosshair). If the focused target is displaced from center, the lens has prism at the point of measurement. For a basic sphere power reading, ensure you are measuring at the optical center of the lens, where the target should be centered.

To find the optical center:

• Move the lens on the stop until the focused target lines are perfectly centered on the reticle
• This position corresponds to the optical center of the lens
• Use the ink-marking device to dot the optical center for later reference

Sphere Power in Minus Cylinder Form

When measuring a lens that has both sphere and cylinder power, the sphere power reading is the first power drum setting that brings one set of lines into focus (in minus cylinder convention). The convention used on the ABO exam and in most U.S. practices is:

• The more plus (or less minus) reading is the sphere power
• The more minus (or less plus) reading is the sphere + cylinder power
• The difference is the cylinder power (always expressed as a minus value in minus cyl form)

This topic is covered in detail under Lensmeter: Cylinder & Axis, but understanding that the sphere reading is the first clear focus point (the more plus reading in minus cyl form) is essential context.

Practical Tips for Accurate Readings

• Turn the power drum slowly and smoothly. Overshooting the focus point and having to reverse is common but leads to less precise readings.
• Ensure the lens is clean. Fingerprints and debris can scatter light and blur the target.
• For strong plus lenses, the target image may be large and slightly fuzzy at the edges. Focus on the center of the target for the sharpest reading.
• For strong minus lenses, the target image will be small. Ensure you are not confusing the target edges with the reticle.

Clinical Relevance

Accurate sphere power measurement is the first step in verifying every prescription. If the sphere is wrong, the entire prescription is wrong. Consistent technique, proper eyepiece focusing, and careful power drum reading ensure reliable verification every time.

Key Takeaways

• A spherical lens brings both sets of target lines into focus simultaneously
• The power drum reads the sphere power directly in diopters (plus or minus)
• Always focus the eyepiece before measuring and center the target on the reticle
• If the target lines focus at different power drum settings, the lens has cylinder power
• In minus cylinder form, the sphere is the more plus of the two readings
• Rotating the axis wheel has no effect on a purely spherical lens