Bifocal lenses are a convenient, alternative to having two pairs of single vision glasses. They are divided into two parts to accommodate a combination of any two different prescriptions into one lens. The upper section being for distance or an intermediate focal length, while the lower section is generally for near vision requirements such as reading.
The upper half of the lens is for distance vision
The lower half of the lens is for looking at things that are near.
HOW DO BIFOCALS WORK?
A bifocal lens has two different areas of vision, divided by a horizontal line across the lens. The top portion helps with distance vision, while the bottom section handles close-up tasks such as reading. The sizes of the two sections may vary according to how much distance or close-up work your eyes especially do.