Designed to correct two distances, generally one for distance vision and one for near vision. The separation of the far and near zone is visible as shown in the graph. Used especially by people with presbyopia (tired eyesight due to age).
The two most common types of bifocals are: Flat Top e Invisibles:
Bifocal Flat Top: the segment designed to correct near vision is crescent-shaped. The far and near segments are separated by a visible line.
Bifocal invisible: the far and near segments are separated by a line that is not visible. The dividing line has been polished in such a way that it is not visible. The vision in the area where the two segments meet may be slightly blurred, causing some difficulty in adaptation.