3.4 Vertical Imbalance and Slab-Off

The near gaze problem

When a patient reads through the lower portion of a lens, the eyes are looking below the distance optical centers or distance reference points. Prentice's rule predicts vertical prism at that lower point. If both lenses have the same vertical meridian power, the induced vertical prism is similar in both eyes. If the lens powers are very different, the vertical prism differs, and the patient may experience vertical imbalance.

Vertical imbalance is especially important for anisometropia, which means a significant refractive difference between the two eyes. It can also appear with antimetropia, where one eye is plus and the other is minus. The patient may do well at distance but struggle at near because the reading gaze passes through a different part of the lenses.

Vertical Imbalance Workflow

1. Identify the reading level below the distance optical center or reference point.
2. Convert that distance to centimeters before using Prentice's rule.
3. Determine the vertical meridian power for each lens.
4. Calculate induced vertical prism separately for OD and OS.
5. Compare the two vertical prism amounts and base directions.
6. Decide whether the imbalance is clinically meaningful enough to require design changes, slab-off discussion, or prescriber/lab consultation.

Calculating vertical imbalance

Use Prentice's rule at the reading level:

P = cF

Here c is the distance from the distance optical center or relevant reference point to the reading point, measured in centimeters. F is the power in the vertical meridian. In many simplified NOCE questions, the lenses are spherical, so the full sphere power is used. With cylinder, use the vertical meridian power.

Example: OD is -2.00 DS and OS is -6.00 DS. The patient reads 10 mm below the optical centers. Convert 10 mm to 1.0 cm. OD induces 1.0 x 2.00 = 2 prism diopters. OS induces 1.0 x 6.00 = 6 prism diopters. Because both lenses are minus and the reading point is below the optical center, the induced prism in each eye is base down. The difference is 4 prism diopters of vertical imbalance.

The patient does not simply feel 8 prism diopters because the imbalance is the difference between eyes when both vertical bases are in the same named direction. The eye with the stronger minus lens has more base down effect at near.

Plus lenses behave differently

With plus lenses, reading below the optical center induces base up prism because plus lenses induce base toward the optical center. Example: OD is +1.00 DS and OS is +5.00 DS. Reading 8 mm below the optical centers means c = 0.8 cm. OD induces 0.8 x 1.00 = 0.8 base up. OS induces 0.8 x 5.00 = 4.0 base up. The vertical imbalance is 3.2 prism diopters, with more base up in OS.

With antimetropia, vertical effects may be opposite and therefore add as imbalance. If OD is +4.00 and OS is -4.00, reading below the centers induces base up OD and base down OS. At 10 mm below, each eye has 4 prism diopters, and the relative vertical imbalance is 8 prism diopters. This is why antimetropia can be a major near comfort issue.

Slab-off purpose

Slab-off, or bicentric grinding, is used to reduce vertical imbalance at near, especially in multifocal lenses. Traditional slab-off adds base up prism in the lower portion of one lens. It is commonly applied to the more minus lens or the less plus lens because that is the lens that needs base up compensation in many classic anisometropic cases.

The slab line is usually placed at or near the top of the segment in a lined multifocal, creating different prism behavior in the lower reading portion. The distance portion remains usable while the near portion receives compensating prism. In a finished lens, the slab line may be visible depending on design and material.

Reverse slab-off can add base down prism and may be used in certain plus-lens imbalance situations, especially with some progressive or specialty designs. For NOCE Basic, the essential point is the reason for slab-off: it compensates vertical imbalance at near. Do not treat slab-off as a solution for ordinary blur, wrong add power, or frame style complaints.

Worked slab-off estimate

Suppose OD is -1.00 DS and OS is -5.00 DS. The patient reads 12 mm below the optical centers. Convert 12 mm to 1.2 cm. OD near prism is 1.2 x 1.00 = 1.2 base down. OS near prism is 1.2 x 5.00 = 6.0 base down. The imbalance is 6.0 - 1.2 = 4.8 prism diopters.

A slab-off order may compensate approximately 4.8 prism diopters in the stronger minus lens, depending on lab rules, material, design limits, and prescriber instructions. The optician's role is to recognize the problem, calculate or communicate the imbalance, and ensure the lab order matches the prescribed or intended solution.

Troubleshooting symptoms

A vertical imbalance patient may report that distance is acceptable but reading is uncomfortable. Words may separate vertically, swim, or require head tilting. The patient may close one eye for near work. They may have difficulty with computer-to-paper transitions or complain that bifocals feel unusable even when the add power measures correctly.

Before assuming slab-off is needed, verify fitting height, segment height, optical center position, prescription, add power, and frame adjustment. A bifocal set too low can force excessive downward gaze. A frame sliding down can increase the distance to the reading point. A wrong vertical OC can create symptoms even in single vision.

Exam traps

Do not calculate vertical imbalance using total lens power if the vertical meridian is different because of cylinder. The meridian of gaze matters.

Do not add same-direction vertical prism between eyes when the question asks imbalance. If both eyes are base down, compare the amounts. If one is base up and the other base down, the relative imbalance adds.

Do not say slab-off fixes all anisometropia. It specifically addresses vertical prism imbalance at near. Other anisometropia issues may include magnification differences, adaptation, lens thickness, cosmetic imbalance, or image size problems.