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A reverse slab-off is a special grinding process used by an optical lab to fix a problem called vertical imbalance in a pair of bifocal lenses. It involves creating a very precise, invisible prism in the reading area of one lens. The purpose of this is to prevent the double vision that can occur when a person with very different prescriptions in each eye looks down to read.
read more about reverse slab off prism ...This problem happens when your two eyes have significantly different prescription powers. When you look down through the reading part of your bifocals, each lens creates a different amount of prism. Your brain sees two images at different heights and cannot merge them into one. This causes double vision, headaches, and eye strain, but only when you are reading.
A reverse slab-off corrects this problem by adding a balancing prism to one of the lenses. The lab carefully grinds this prism onto the front surface of the lens that has the most plus power. This added prism cancels out the imbalance from the other lens. As a result, both of your eyes see the image at the same height, which eliminates the double vision and allows for comfortable reading.
A reverse slab-off is one of two ways to fix vertical imbalance. The other method is a "traditional slab-off." Both techniques solve the exact same problem, and the final visual result for the wearer is identical. The choice of which method to use is a technical decision made by the optical lab based on the specific prescription and lens type.
While both traditional slab-off and reverse slab-off achieve the same optical result, the reverse method is often preferred for cosmetic reasons. A traditional slab-off creates a visible horizontal line across the lens that can look like a scratch or a bifocal line. A reverse slab-off is molded onto the blank or cut digitally, often resulting in a much fainter line that is harder for others to see when looking at you.
Yes, if you look very closely, you will see a very faint, thin horizontal line that runs across the entire lens. This line is usually placed at the same height as the top of your bifocal segment so it is not very noticeable.
Vertical imbalance only happens when you look down, away from the optical center of your lenses. When you look straight ahead for distance, your eyes are looking through the part of the lens where there is no imbalance, so your vision is single and clear.
Yes. Some people prefer to avoid the problem by using two separate pairs of glasses, one for distance and one for reading. Wearing contact lenses also eliminates the problem, as the lenses move with your eyes so you are always looking through the center.
You should immediately talk to your eye doctor if you get a new pair of bifocals and experience double vision, headaches, or intense eye strain only when you try to read. These are classic signs of vertical imbalance. Your doctor can confirm the problem and order a new lens with a slab-off to make your reading vision comfortable again.
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Understanding Multifocal Designs The Role of Slab-off. Optical Training. https://opticaltraining.com/html/continuing_ed/wbt/ABO/Lens_Materials_Coatings/page_sixteen.html. Accessed April 7, 2026.
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