Measuring Cylinder Power and Axis
When a lens corrects astigmatism, it has different powers in different meridians. On the lensmeter, this means the two sets of target lines (mires) do NOT focus simultaneously. Instead, each set focuses at a different power drum setting. The difference between these two readings is the cylinder power, and the orientation of the axis wheel when the lines are aligned gives the axis.
Understanding the Two Principal Meridians
An astigmatic lens has two principal meridians oriented 90 degrees apart. Each meridian has its own power. When you look through the lensmeter:
- One set of lines (typically the thin, continuous lines) focuses at one power drum setting
- The other set of lines (typically the thick, broken lines) focuses at a different setting
The axis wheel must be rotated to align the target lines properly before reading the power. Correct axis alignment is indicated when the target lines appear sharp, straight, and unbroken.
Step-by-Step Measurement Process
- Focus the eyepiece (always the first step)
- Place the lens on the lens stop, back surface toward you
- Rotate the axis wheel until one set of lines appears continuous and unbroken. If the lines appear broken or skewed, the axis setting is incorrect.
- Focus the first set of lines: Turn the power drum until the first set of lines (the more plus reading in minus cylinder form) is perfectly sharp. Record this as the sphere power.
- Focus the second set of lines: Continue turning the power drum (toward more minus) until the other set of lines comes into sharp focus. Record this reading.
- Calculate cylinder power: Subtract the first reading from the second reading. The result (a minus value) is the cylinder power.
- Read the axis: The axis is read from the axis wheel at the position where the first set of lines (sphere meridian) was aligned.
Worked Example
| Step | Reading |
|---|---|
| Axis wheel aligned to clear lines | 90 degrees |
| First focus (thin lines sharp) | -2.00 D |
| Second focus (thick lines sharp) | -3.50 D |
| Sphere power | -2.00 D |
| Cylinder power | -3.50 - (-2.00) = -1.50 D |
| Axis | 90 degrees |
| Final Rx | -2.00 -1.50 x 090 |
Axis Alignment Tips
Correct axis alignment is critical for accurate measurement. Signs that the axis is properly set:
- The target lines appear perfectly straight and continuous
- There is no break or stair-step pattern in the lines
- Fine-tuning the axis wheel while the lines are in focus produces the sharpest image
Signs that the axis is incorrect:
- Lines appear broken or stair-stepped
- Lines appear blurry even when the power drum setting should bring them into focus
- You cannot get both sets of lines clear at any power drum setting
Minus Cylinder vs. Plus Cylinder Convention
The same lens can be expressed in either minus cylinder or plus cylinder form. The ABO exam uses minus cylinder convention:
- Minus cylinder: Sphere is the more plus reading; cylinder is expressed as a negative value
- Plus cylinder: Sphere is the more minus reading; cylinder is expressed as a positive value (used by some ophthalmologists)
To convert between forms, use the transposition rule:
- Add the sphere and cylinder algebraically to get the new sphere
- Change the sign of the cylinder
- Change the axis by 90 degrees
Example: -2.00 -1.50 x 090 converts to -3.50 +1.50 x 180
High Cylinder Measurements
High cylinder powers (above 2.00 D) require extra care:
- The two focus points are far apart on the power drum, making it harder to keep track of readings
- The target may appear very distorted between the two focus points
- Axis accuracy becomes more critical since even a small axis error produces noticeable visual distortion for the patient
For cylinders above 2.00 D, the ANSI standard allows a tighter axis tolerance (closer to the prescribed axis) than for lower cylinders, reflecting the increased sensitivity to axis error.
Clinical Relevance
Accurate cylinder and axis measurement is essential for verifying prescriptions. An axis error of even 5 degrees in a high-cylinder prescription can cause significant blurriness and patient discomfort. Proper technique, careful axis alignment, and systematic reading of the power drum ensure that dispensed lenses match the prescribed correction.
Key Takeaways
- Astigmatic lenses cause the two sets of target lines to focus at different power drum settings
- The axis wheel must be aligned first, then the power drum adjusted for each meridian
- In minus cylinder form: sphere = more plus reading, cylinder = difference (negative), axis = sphere meridian orientation
- Lines that appear broken or stair-stepped indicate incorrect axis alignment
- High cylinder powers require extra care and tighter axis tolerance
- Transposition converts between minus and plus cylinder forms without changing the actual prescription