Which lens gets the slab-off?

Conventional slab-off is always applied to the lens with more minus power (or less plus power). This lens produces more base-up prism effect during downgaze, so the slab-off adds base-down prism to compensate. Reverse slab-off is applied to the opposite (more plus) lens.

Can slab-off be used with progressive lenses?

Yes, slab-off can be applied to progressive lenses, though it is more technically challenging. Some progressive lens designs include built-in prism compensation (sometimes called prism thinning or R-compensation) that partially addresses vertical imbalance without requiring a separate slab-off grind.

How can you tell if a lens has a slab-off?

Look for a faint horizontal line running across the lens at approximately the seg line level. You can also verify by holding the lens against a straight edge and observing the line displacement, or by measuring with a lensometer in the reading zone to detect the built-in prism.

Slab-Off & Bicentric Grind: Correcting Vertical Imbalance for ABO

What Is a Slab-Off?

A slab-off (also called a bicentric grind) is a specialized lens surfacing technique that corrects vertical prismatic imbalance in multifocal lenses. It creates a second optical center in the reading portion of the lens, effectively adding base-up prism to the lower half of one lens to equalize the vertical prismatic effects between the two eyes.

Think of it as grinding a wedge of prism into the bottom of one lens so that both eyes experience the same amount of vertical displacement when reading.

When Is Slab-Off Needed?

Slab-off is indicated when an anisometropic patient wearing multifocals experiences 1.5Δ or more of vertical imbalance at the reading level. The most common scenario involves patients with a significant power difference between their two eyes in the vertical meridian.

Slab-off is always applied to the lens with MORE minus power (or LESS plus power). This is the eye that produces more base-up prism effect when looking down.

How It Works

During normal downgaze through a minus lens, the prismatic effect is base up (light bends toward the thicker edge, which is at the top of a minus lens). The stronger the minus, the more base-up prism. A slab-off grinds base-down prism into the lower portion of the more-minus lens, neutralizing the excess base-up effect.

The Bicentric Concept

The term "bicentric" means the lens has two optical centers: one for distance viewing and one for near. The distance OC is positioned normally. The near OC is created by the slab-off grind. Between these two centers, you can see a faint visible line across the lens, running approximately at the top of the bifocal segment.

Slab-Off vs. Reverse Slab-Off

Feature	Slab-Off (Conventional)	Reverse Slab-Off
Applied to	More minus (less plus) lens	More plus (less minus) lens
Prism added	Base down in reading zone	Base up in reading zone
Method	Grinding/molding	Molding into semi-finished blank
Visible line	Yes, at seg line level	Less noticeable
Cost	Moderate	Often less expensive

Reverse slab-off is becoming more popular because it can be molded into the semi-finished lens blank, making it smoother and less visible than a ground slab-off. It achieves the same optical result by adding compensating prism to the opposite lens.

Calculating the Slab-Off Amount

The amount of slab-off prism needed equals the vertical imbalance at the reading position:

Determine the reading drop (typically 10 mm = 1.0 cm)
Calculate the vertical meridian power of each lens at the reading point
Find the difference: Slab-off amount = drop (cm) x |power difference|

Example: OD is -1.00 DS, OS is -4.00 DS. Reading drop = 10 mm.

Imbalance = 1.0 × (4.00 - 1.00) = 3Δ

Apply 3Δ slab-off to the OS (more minus) lens.

Identifying Slab-Off in the Lab

You can verify a slab-off by:

Holding the lens at arm's length and looking through the lower portion. You will see a visible demarcation line.
Using a lensometer: neutralize the distance portion, then move to the reading zone. The lensometer will show the slab-off prism amount.
Holding the lens against a straight line. The line will appear to shift at the slab-off boundary.

Applying slab-off to the wrong lens. Always apply conventional slab-off to the MORE MINUS lens. If you apply it to the wrong eye, you double the imbalance instead of correcting it.

Key Takeaways

Slab-off creates a second optical center to correct vertical imbalance in multifocals
Apply conventional slab-off to the more minus (or less plus) lens
Reverse slab-off goes on the more plus lens and may be less visible
The amount of slab-off equals the calculated vertical imbalance
A visible line at the seg level is the telltale sign of a slab-off grind