The upper half of the lens is for distance vision.Your normal glasses prescription will go in this part of the lens.
The lower half, or small segment in the lowerhalf of the lens, is for looking at things that are near. Like built-in reading glasses in your normal prescription.
Types of bifocals lenses
Most of the lens in bifocal glasses will contain the power for distance vision, while only a smart part (or segment)corrects your near vision. The lens segment devoted to nearvision correction can be different shapes, such as:
half of the lens
Standard bifocal lenses - £49
Offering distance and near vision in one lens. Bifocals have a small segment at the lower part of the lens containing your near vision area.
Premium bifocal lenses - £59
As for standard bifocals, these lenses offer distance and near vision in one lens. Premium bifocals have a larger segment at the lower part of the lens containing
your near vision area.