How do you measure add power on a lensmeter?

Measure the distance sphere power through the upper portion of the lens, then slide the lens to measure the sphere power through the near segment. Subtract the distance sphere from the segment sphere. The result is the add power. For example, if the distance reads -2.00 and the segment reads +0.25, the add is +0.25 - (-2.00) = +2.25 D.

Why is my add power calculation giving a negative number?

A negative add power calculation means either the measurements are reversed (you read the segment as the distance or vice versa) or there is a measurement error. The near segment always has more plus power than the distance portion, so the add must always be positive. Re-measure both portions carefully and ensure you are reading through the correct areas of the lens.

What should the intermediate power be in a trifocal?

The intermediate segment of a trifocal should read approximately half the full near add power. If the near add is +2.50 D, the intermediate should measure about +1.25 D. This intermediate power provides clear vision at arm's length (computer or dashboard distance), bridging the gap between the distance and full near corrections.

What is the ANSI tolerance for add power?

According to ANSI Z80.1, the tolerance for add power is ±0.12 D for adds up to +4.00 D. This means a prescribed add of +2.00 is acceptable if it measures between +1.88 and +2.12 on the lensmeter. Add power outside this range should be returned to the lab for rework.

Lensmeter: Measuring Add Power in Multifocal Lenses

What Is Add Power Verification?

Verifying add power on the lensmeter confirms that the near-vision supplement in a multifocal lens matches the prescription. The add power is not measured directly. Instead, it is calculated as the difference between the distance power and the near segment power. This two-step measurement is performed for every bifocal, trifocal, and progressive lens verification.

Step-by-Step Add Power Measurement

Step 1: Measure the Distance Power

Place the lens on the lens stop with the back surface facing you
Position the distance portion (upper part) of the lens on the lens stop
Focus the mires and record the full distance prescription (sphere, cylinder, axis)
Record the sphere value specifically. For add calculation, you need the spherical equivalent of the distance portion.

Step 2: Measure the Segment (Near) Power

Without removing the lens, slide it so the near segment (the lower portion) is centered on the lens stop
Focus the mires through the near segment
Record the sphere value through the segment

Step 3: Calculate the Add Power

Add Power = Segment Sphere - Distance Sphere

Example	Distance Sphere	Segment Sphere	Add Power
Example 1	-3.00 D	-1.00 D	-1.00 - (-3.00) = +2.00 D
Example 2	+1.50 D	+3.75 D	+3.75 - (+1.50) = +2.25 D
Example 3	-0.50 D	+1.50 D	+1.50 - (-0.50) = +2.00 D

The add power should always be a positive value. If your calculation yields a negative number, check your measurements and calculation. The near segment always has more plus power than the distance portion.

The add power is the same regardless of the distance prescription. A patient with -5.00 distance and a patient with +2.00 distance can both have a +2.00 add. The add represents the supplemental plus power for near vision, independent of the distance correction.

Locating the Segment

For bifocals, the segment is visually obvious. A distinct line separates the distance and near portions. Slide the lens until the segment area is centered on the lens stop.

For trifocals, there is both an intermediate and a near segment. The intermediate segment should read approximately half the near add. For example, if the near add is +2.50, the intermediate should read close to +1.25. Measuring the intermediate confirms the trifocal is correctly fabricated.

For progressive lenses, there is no visible line. Locating the correct measurement points requires using the lens engravings. This is covered in detail under Lensmeter: Progressive Verification.

When measuring the segment power of a lined bifocal, position the lens so that you are reading through the center of the segment, not near its edge. Power readings near the segment edge may be affected by the transition zone and can give inaccurate results.

Cylinder in the Near Segment

In most multifocal lenses, the cylinder power and axis should be the same in both the distance and near portions. When you measure through the near segment, you should find the same cylinder and axis as the distance measurement, with only the sphere changing by the add amount.

If the cylinder or axis changes significantly between the distance and near portions, this may indicate:

A fabrication error
The segment is not properly aligned
You are measuring at an incorrect point on the lens

Comparing the total near power to the add power. The total near power includes the distance prescription plus the add. A patient with -3.00 distance and +2.00 add has a total near power of -1.00, not +2.00. The add itself is only +2.00. Do not confuse the two values.

ANSI Tolerances for Add Power

According to ANSI Z80.1, the add power tolerance is ±0.12 D for adds up to +4.00 D. This is a tight tolerance, reflecting the importance of accurate near-vision correction for patient comfort. If the measured add falls outside this range, the lens should be returned to the lab for rework.

Verifying Both Eyes

Always verify the add power in both lenses of a pair. While the prescribed add is almost always the same for both eyes, fabrication errors can cause one lens to have the correct add while the other is off. Compare both to the prescription before approving the job.

Clinical Relevance

An incorrect add power is one of the most common reasons for patient complaints about near vision. Too little add and the patient cannot read comfortably; too much add and the working distance is uncomfortably short. Accurate add verification on the lensmeter catches fabrication errors before the glasses reach the patient.

Key Takeaways

Add power = Segment sphere - Distance sphere (always a positive result)
Measure the distance power first, then slide the lens to measure through the segment
The add power should be the same for both lenses unless specifically prescribed otherwise
In trifocals, the intermediate segment should read approximately half the near add
The cylinder and axis should be the same in both the distance and near portions
ANSI tolerance for add power is ±0.12 D