What is normally ground on the lens for a standard slab off?

A standard slab-off is a technique used in lens manufacturing to alleviate the distortions caused by a significant difference in prescription strength between the two eyes, often in cases of anisometropia. This procedure is typically applied to bifocal lenses where one eye may require more correction than the other, particularly when a higher degree of myopia is present in one eye compared to the other.

When creating a slab-off, the lens that requires it—often the lens with the most minus power—has a section ground into it at a specific meridian. For standard slab-offs, the most effective way to balance the vision and ensure the lenses provide the best correction with minimal distortion is to grind the most minus power at 90 degrees. This is because minus lenses have more curvature when viewed at this meridian, allowing for an effective transformation in lens thickness to counteract visual discrepancies between the two eyes.

In this case, grinding the most minus power at 90 degrees achieves the goal of reducing the overall effective power of the lens in that meridian while keeping the optical characteristics intact and ensuring that the wearer has clear vision.