What is the difference between the sphere and cylinder mires on a lensometer?

The sphere mires are typically the thinner lines (or dots) in the target pattern, while the cylinder mires are the thicker lines (or dashes). When measuring a lens with both sphere and cylinder power, these two sets of lines focus at different power settings. The first reading (sphere lines) gives the sphere power, and the difference between the two readings gives the cylinder power.

Why is it important to focus the eyepiece before measuring?

The lensometer eyepiece must be focused to your own eye so the reticle crosshair appears sharp. If the eyepiece is not properly focused, your own refractive error will be added to or subtracted from the measurement, producing an inaccurate reading. This is similar to how adjusting binocular eyepieces is necessary for a clear view.

What does it mean if both sets of mire lines focus at the same power?

If both the sphere and cylinder mires focus at the same power setting, the lens is purely spherical with no cylindrical component. The prescription would be recorded as sphere only, such as -2.00 DS (diopters sphere) with no cylinder value.

Lensometer Principles and Use: Measuring Spectacle Lens Power

What Is a Lensometer?

The lensometer (also called a focimeter or lensmeter) is one of the most frequently used instruments in an ophthalmic practice. It measures the optical power of spectacle lenses, including the sphere, cylinder, and axis components. Every time a patient brings in their current glasses, the lensometer tells you exactly what prescription they are wearing.

How the Lensometer Works

The instrument projects a target pattern called mires through the spectacle lens being measured. By adjusting internal lenses within the lensometer, the operator brings the mires into sharp focus. The readings on the instrument's dials reveal the lens power at the point of neutralization.

Think of it as a puzzle: the spectacle lens blurs the mires in a specific way that tells you its power. By adding the exact opposite power inside the lensometer, you "cancel out" the blur, and the mires snap into clear focus.

Mire Patterns

Most lensometers display two sets of lines in the mire pattern:

Sphere lines (thin lines or dots): These focus first when adjusting the power wheel. The reading at which these lines are sharp gives the sphere power.
Cylinder lines (thick lines or dashes): These focus at a different power if cylinder is present. The difference between the two readings gives the cylinder power.

If both sets of lines focus at the same power setting, the lens is purely spherical (no cylinder). If they focus at different settings, the lens has spherocylindrical power, and the difference equals the cylinder amount.

Step-by-Step Neutralization

Clean the lens and place it on the lensometer stage with the back (concave) surface facing you
Focus the eyepiece by adjusting it until the reticle (crosshair) is sharp, before placing any lens
Center the lens on the nose rest so the optical center aligns with the lensometer aperture
Rotate the power wheel to focus the sphere mires (thin lines) first. Record this as the sphere power.
Rotate the axis wheel until the cylinder mires (thick lines) are properly oriented
Continue adjusting the power wheel to focus the cylinder mires. The difference between this reading and the sphere reading is the cylinder power.
Read the axis from the axis wheel at the point where the cylinder mires are sharp

Always focus the sphere lines first (the first set of lines to come into focus as you move from high plus toward minus). This gives you the most plus or least minus sphere, which is the standard for recording prescriptions in minus cylinder form.

Minus Cylinder vs. Plus Cylinder Form

The same prescription can be written in two forms:

Minus cylinder form (standard in ophthalmology): Focus the sphere lines first (more plus reading), then the cylinder lines (more minus reading). The cylinder value is negative.
Plus cylinder form (standard in optometry): Focus the cylinder lines first (more minus reading), then the sphere lines (more plus reading). The cylinder value is positive.

Both forms describe the same optical correction. For the COA exam and in ophthalmology practices, minus cylinder form is standard.

Tips for Accurate Readings

Always focus the eyepiece first to avoid parallax error
Ensure the lens is flat against the stage and not tilted
Read the glasses for the right eye first, then the left (this is standard convention)
Check for prism by noting whether the mire target is centered on the reticle at the optical center
Verify your reading makes clinical sense (compare to the patient's known history or symptoms)

Forgetting to focus the lensometer eyepiece before measuring. An unfocused eyepiece introduces your own refractive error into the reading, producing inaccurate measurements. Always adjust the eyepiece until the reticle crosshair is sharp before placing any lens on the stage.

Key Takeaways

The lensometer measures sphere, cylinder, and axis of spectacle lenses by neutralizing mire patterns
Sphere mires (thin lines) are focused first, followed by cylinder mires (thick lines) for minus cylinder form
The difference between the two readings equals the cylinder power
Always focus the eyepiece reticle before measuring and ensure the lens sits flat on the stage
Minus cylinder form is standard in ophthalmology: focus the more plus reading first