Slab-Off Lenses: Correcting Vertical Imbalance in Anisometropic Prescriptions

What Is a Slab-Off Lens?

A slab-off is a specialized lens modification used to correct vertical imbalance at near viewing distances. When a patient has significantly different prescriptions between their two eyes (a condition called anisometropia), each lens induces a different amount of vertical prism when the patient looks down to read. This prismatic difference can cause diplopia (double vision), eye strain, and discomfort during near work.

The slab-off is a bicentric grind applied to the lower portion of one lens. The result is a visible line across the lens where the grind begins, similar in appearance to a bifocal line. Above this line, the lens has its standard distance power. Below the line, additional vertical prism has been ground in to compensate for the imbalance.

Why Vertical Imbalance Occurs

When a patient looks through the optical center of their lenses, there is zero induced prism (per Prentice's Rule). However, when the eyes rotate downward to read, they look through a point below the optical center. According to Prentice's Rule:

Prism (in prism diopters) = Decentration (in cm) x Lens Power (in diopters)

If both lenses have similar power, the induced vertical prism is roughly equal in both eyes and the brain compensates easily. But if the powers differ significantly, one eye experiences more induced prism than the other, creating a vertical imbalance.

A vertical imbalance of 1.5 prism diopters or more generally requires compensation. Most patients can tolerate up to about 1.0 to 1.5 PD of vertical imbalance through fusional vergence reserves.

Conventional Slab-Off

A conventional slab-off adds base-up prism to the lower portion of one lens. It is applied to the lens with the most minus power (or least plus power) in the vertical meridian. This is because the more minus lens induces the most base-down prism when the patient looks down, so adding base-up prism in the reading area neutralizes the excess.

How to remember which eye gets the slab-off:

• Calculate the vertical power in the 90-degree meridian for each eye
• Identify which eye has the more minus (or less plus) power
• Apply the conventional slab-off to that eye

Reverse Slab-Off

A reverse slab-off adds base-down prism to the lower portion of the lens. It is applied to the eye with the most plus power (or least minus power) in the vertical meridian. The end result is the same as a conventional slab-off: the vertical imbalance between the two eyes is reduced or eliminated at the reading position.

Reverse slab-off is sometimes preferred because:

• It can be easier to fabricate on certain lens designs
• It may produce a less noticeable line on the lens
• Some free-form lens systems generate it digitally

Slab-Off vs. Other Solutions

Slab-off is not the only way to address vertical imbalance. Alternatives include:

• Prescribed prism: Adding vertical prism to one or both lenses, but this affects distance vision too
• Dissimilar segment heights: Raising the seg height on one side to move the reading zone closer to the OC, reducing induced prism
• Contact lenses: Since contacts move with the eye, they do not induce differential prism. Contacts eliminate the problem entirely for eligible patients.
• Separate reading glasses: A single-vision reading pair can be designed to minimize imbalance

Identifying a Slab-Off on the Lensmeter

When verifying a lens with a slab-off, you will notice a visible line across the lens similar to a bifocal line. Measuring the power above and below this line reveals a change in vertical prism. The prism difference equals the slab-off amount. During verification:

• Measure the distance power above the slab-off line
• Measure the power through the lower portion
• Note the vertical prism difference
• Confirm it matches the prescribed compensation amount

Clinical Relevance

Slab-off lenses solve a specific and uncomfortable problem for anisometropic patients wearing multifocal lenses. Without correction, these patients may tilt their head, close one eye, or avoid reading altogether. Recognizing when vertical imbalance is the source of a patient's near-vision complaints demonstrates advanced clinical skill.

Key Takeaways

• Slab-off corrects vertical imbalance caused by different lens powers between the two eyes
• Conventional slab-off adds base-up prism to the most minus lens
• Reverse slab-off adds base-down prism to the most plus lens
• Vertical imbalance above 1.5 PD generally warrants correction
• Prentice's Rule calculates the induced prism at any point below the optical center
• Contact lenses eliminate differential prism entirely since they move with the eye