The upper and lower portions in common type of bi-focal lenses are respectively

Bifocal lenses are designed to correct vision for individuals suffering from presbyopia, often in combination with myopia or hypermetropia. A common bifocal lens consists of two distinct optical segments. The upper portion is designed for distance vision and is typically a concave lens to correct myopia (short-sightedness). The lower portion is designed for near-vision tasks, such as reading, and is a convex lens to provide the necessary convergence for close-up focus. This dual-segment design allows the wearer to look through the top part for distant objects and the bottom part for near objects without changing spectacles. While modern progressive lenses offer a gradual transition, the traditional bifocal maintains a clear dividing line between these two specific lens types.