How do you calculate add power from a lensometer?

Measure the sphere power in the distance zone, then measure the sphere power in the near zone. Subtract the distance reading from the near reading. Add power = near sphere - distance sphere. The result is always a positive number because the near zone adds converging power. For example, if distance reads -2.00 and near reads -0.25, the add is +1.75 D.

What are ANSI tolerances for add power in multifocal lenses?

ANSI Z80.1 specifies that add power should be within ±0.12 D of the prescribed value for add powers up to +4.00 D. If the measured add deviates beyond this tolerance, the lens does not meet the standard and should be remade. This tight tolerance reflects the significance of add power accuracy for comfortable near vision.

How do you find the correct zones on a progressive lens in the lensometer?

Use the manufacturer's hidden engravings (small circles, dots, or alphanumeric codes) as reference points. Manufacturer lens charts or progressive identifiers show the location of the fitting cross, distance reference point, and near reference point relative to these engravings. Digital lensometers often have a progressive scan mode that automatically identifies the correct zones as you move the lens across the aperture.

Lensometer: Multifocal and Progressive Lenses: CPO Exam Guide

Verifying Multifocal Lenses

Verifying multifocal and progressive lenses requires additional steps beyond standard single vision lens measurement. As a CPO involved in dispensing, accurately reading add power and understanding segment positioning is essential for ensuring the patient receives the correct prescription.

Bifocal Lens Verification

A bifocal lens has two distinct optical zones. To fully verify a bifocal:

Step 1: Measure the Distance Portion

Place the lens with the distance zone in the lensometer aperture
Find sphere, cylinder, and axis for the distance prescription
Mark the optical center of the distance zone

Step 2: Measure the Near Segment

Slide the lens so the near segment is in the aperture
Focus the target in the near zone and read the sphere
The near segment typically shows only sphere power (no additional cylinder in most bifocals)

Step 3: Calculate Add Power

Add power = Near sphere reading - Distance sphere reading

Add power is always positive. For example, if the distance reads -2.00 sphere and the near reads -0.25, the add power is +1.75 D.

Add power should always be a positive number. If you calculate a negative add, recheck your readings. A negative add indicates an error in positioning or reading, since the near zone always adds converging power to the distance correction.

Progressive Lens Verification

Progressive lenses require careful positioning because the power changes gradually across the lens surface. To find the correct measurement zones, use the manufacturer's hidden engravings or a progressive lens identifier.

Locating Reference Points

Prism reference point (PRP): a small circle or cross engraved on the lens where any prescribed prism is measured
Distance reference point: typically 4 mm above the fitting cross (the point that aligns with the pupil)
Near reference point: typically 12-14 mm below the fitting cross
Fitting cross: the small cross marking on the lens that aligns with the center of the pupil at the dispensing appointment

Many lensometers have a progressive lens mode that automatically finds the distance and near zones when you scan the lens across the aperture. In manual mode, use a progressive lens chart (available from lens manufacturers) that shows where the reference points are located relative to the hidden engravings for that specific product.

ANSI Tolerances for Add Power

The ANSI Z80.1 standard specifies acceptable tolerances for add power verification:

Add power tolerance: ±0.12 D for add powers up to +4.00 D
If the measured add differs from the prescribed add by more than this tolerance, the lens should be remade

Segment Height Verification

Segment height is the distance from the bottom of the frame to the top of the near segment (bifocal) or fitting cross (progressive). Verify it using a ruler or PD stick with the lenses in the frame, confirming it matches the ordered height within 1 mm.

Measuring add power from the wrong zone of a progressive lens. If you measure from the intermediate zone rather than the true near reference point, you will read a lower add power than actually exists. Use manufacturer lens charts or the progressive mode on a digital lensometer to locate the correct near zone.

Key Takeaways

Add power = near zone reading minus distance zone reading (always positive)
ANSI tolerance for add power: ±0.12 D
Progressive lens verification requires locating the distance, near, and prism reference points using manufacturer engravings
Fitting cross must align with the patient's pupil at dispensing
Segment height tolerance: within 1 mm of ordered height