10.2 Verifying Cylinder Axis & Power

Understanding Spherocylindrical Optics

A spherocylindrical lens is used to correct astigmatism, a condition where the eye's cornea or crystalline lens has unequal curvature in different meridians. Consequently, a spherocylindrical lens has two principal meridians of refractive power: the sphere meridian (which represents the baseline spherical correction) and the cylinder meridian (which represents the maximum difference in power from the sphere meridian). These two meridians are always perpendicular (90 degrees apart) to each other.

Because the power varies across the lens, a manual lensmeter will not focus both sets of target lines simultaneously. Instead, the thin (sphere) lines will focus at one setting on the power wheel, and the thick (cylinder) lines will focus at a different setting. The difference between these two readings represents the cylinder power, while the orientation of the meridians is determined by the axis wheel positioning.

The Spherocylindrical Target Appearance

When a spherocylindrical lens is placed in a manual lensmeter, the target lines will initially appear broken, distorted, or "stepped." This occurs because the target lines are not aligned with the principal meridians of the lens. The optician must perform cylinder line alignment by rotating the axis wheel until the target lines form continuous, straight, and unbroken lines.

• Thin lines (sphere lines) represent the first principal meridian. When they are aligned and in focus, they appear as three continuous parallel lines.
• Thick lines (cylinder lines) represent the second principal meridian. When they are in focus, they appear as three continuous parallel lines perpendicular to the thin lines.

If the lines are aligned but out of focus, they will appear blurry but straight. If they are misaligned, they will appear broken or stepped even when they are in focus.

Step-by-Step Neutralization in Minus Cylinder Form

In ophthalmic dispensing, prescriptions are standardly written in minus cylinder form (where the cylinder value is negative). To neutralize a lens in minus cylinder form, the optician must follow a specific sequence. This ensures that the sphere power is measured first and the cylinder power is subtracted from it, yielding a negative cylinder value.

The step-by-step procedure is:

1. Place and Align the Lens: Place the spectacles on the lens table with the back surface against the lens stop. Clamp the lens securely.
2. Locate the Sphere Meridian: Turn the power wheel to a high plus setting (blurring the target). Slowly rotate the power wheel in the minus direction (reducing plus power or increasing minus power) until one set of target lines begins to clear.
3. Perform Axis Alignment: If the lines appear broken or stepped, rotate the axis wheel until the lines align into continuous, unbroken lines. Continue adjusting the power wheel in the minus direction until these lines are in sharp, crisp focus.
4. Verify Sphere Line Focus: To ensure the neutralization is in minus cylinder form, the lines that focus first (at the more positive/less negative setting) must be the thin lines (sphere lines). If the thick lines focus first, rotate the axis wheel by 90 degrees and refocus the thin lines.
5. Record the Sphere Power: Look at the power wheel scale. This first reading is the sphere power.
6. Locate the Cylinder Meridian: Continue turning the power wheel in the minus direction (more negative/less positive). The thin lines will become blurry, and the thick lines (cylinder lines) will begin to clear. Adjust the power wheel until the thick lines are in sharp, crisp focus.
7. Record the Second Reading: Look at the power wheel scale. This is the second reading.
8. Calculate the Cylinder Power: The cylinder power is the power wheel reading difference between the second reading and the first reading (Second Reading - First Reading). Because you moved the wheel in the minus direction, the second reading will always be more negative than the first, resulting in a negative cylinder power.
9. Read the Axis: Look at the axis wheel. The number indicated on the drum is the cylinder axis.

Neutralization Example Calculations

To illustrate this process, consider the following three worked examples of spherocylindrical neutralization in minus cylinder form:

Example 1: High Plus Prescription

• First Reading (Thin Lines focus): +4.50 D
• Second Reading (Thick Lines focus): +2.25 D
• Axis Wheel Reading: 090°
• Calculation: The sphere power is +4.50 D. The cylinder power is the difference: (+2.25 D) - (+4.50 D) = -2.25 D.
• Resulting Rx: +4.50 -2.25 x 090

Example 2: Mixed Astigmatism

• First Reading (Thin Lines focus): +1.50 D
• Second Reading (Thick Lines focus): -1.00 D
• Axis Wheel Reading: 180°
• Calculation: The sphere power is +1.50 D. The cylinder power is the difference: (-1.00 D) - (+1.50 D) = -2.50 D.
• Resulting Rx: +1.50 -2.50 x 180

Example 3: High Minus Prescription

• First Reading (Thin Lines focus): -2.00 D
• Second Reading (Thick Lines focus): -5.50 D
• Axis Wheel Reading: 045°
• Calculation: The sphere power is -2.00 D. The cylinder power is the difference: (-5.50 D) - (-2.00 D) = -3.50 D.
• Resulting Rx: -2.00 -3.50 x 045

Common Errors in Cylinder Verification

A common mistake is neutralizing the lens in plus cylinder form. If the optician focuses the thick lines first (at a more positive power) and then turns the wheel in the minus direction to focus the thin lines, the cylinder power calculation will yield a positive value. While mathematically equivalent after transposition, clinical standards require documenting prescriptions in minus cylinder form. If this occurs, the optician should transpose the result: add the sphere and cylinder powers to find the new sphere power, change the sign of the cylinder to negative, and rotate the axis by 90 degrees.