Vision correction usually involves standard lens grinding, but complex visual conditions require specialized techniques. When a patient needs a multifocal lens (bifocal or progressive), an eye care professional may recommend a modification called a slab off. This precise lens fabrication addresses a specific vertical misalignment issue that standard prescriptions cannot resolve, ensuring clear, comfortable vision.
Defining the Slab Off Procedure
The term “slab off” refers to a lens modification technique also known as bicentric grinding or compensating prism. This specialized process is applied exclusively to the lower, near vision portion of a multifocal lens. The primary purpose is to introduce a precisely calculated amount of vertical prism into one lens. This added prism neutralizes an unwanted prismatic effect the wearer would otherwise experience. The procedure involves grinding material from the lens surface, creating a second optical center in the reading area.
The Root Cause: Vertical Prism Imbalance
The need for a slab off arises from vertical prism imbalance, a consequence of having significantly different prescriptions in each eye. This difference in refractive error is medically termed anisometropia. While a person with anisometropia can see clearly looking straight ahead through the distance portion of their lenses, problems begin when they shift their gaze downward to read.
When the eyes look down, they move away from the optical center, inducing a prismatic effect in both lenses. Because the lens powers are unequal due to anisometropia, the prismatic effect in each eye is also unequal, creating the vertical imbalance. A difference in prescription of 1.5 diopters or more between the two eyes is often the threshold where this imbalance becomes clinically significant.
This unequal vertical prism displaces the image seen by one eye relative to the other. The brain receives two images that are not vertically aligned, making it difficult to fuse them into a single, clear picture. Symptoms often include chronic eye strain, headaches, and sometimes double vision, particularly when reading. The slab off procedure restores comfortable binocular vision in the reading field.
How the Slab Off Corrects Vision
The slab off introduces an opposing prism into one lens to counteract the unwanted vertical prism created by anisometropia. This is achieved by grinding a base-up prism into the lower half of the lens that naturally induces the most base-down prism when reading. This compensating prism shifts the image in the affected eye so it vertically aligns with the image from the fellow eye, resolving the visual conflict.
For a conventional slab off, a base-up prism is ground onto the back surface of the lens with the stronger minus or weaker plus prescription. The alternative, the reverse slab, introduces a base-down prism and is applied to the lens with the weaker minus or stronger plus prescription. In both cases, the goal is to equalize the total prismatic effect at the reading depth, allowing the eyes to comfortably fuse the images.
The physical result of this grinding technique is a distinct, horizontal line etched across the lens surface, typically positioned just above the reading segment. This line is a cosmetic artifact of the manufacturing process and does not interfere with corrected vision. Even in modern progressive lenses, the bicentric grinding process leaves this subtle horizontal demarcation. The line confirms that the specialized compensation has been accurately applied, ensuring images are properly re-aligned for near tasks.
Practical Considerations and Adaptation
For the wearer, the slab off often provides an immediate improvement in comfort, as the correction allows for comfortable binocular fusion. While an adjustment period is necessary, it is usually spent getting used to the peripheral distortion of a new prescription, not adapting to the slab off itself. The resolution of long-standing discomfort, eye strain, and double vision is typically a positive experience for the patient.
The slab off technique requires specialized laboratory work and precision grinding, adding complexity and cost to lens production. Because of this added expense, eye care professionals sometimes explore alternatives for managing vertical imbalance. These alternatives include prescribing contact lenses, which minimize prismatic effects, or recommending two separate pairs of single-vision glasses (one for distance and one for reading). However, the slab off remains an effective and convenient solution, allowing the wearer to use a single pair of multifocal glasses for all visual tasks without vertical misalignment.