 There are many different shapes, sizes, and positions for the add segment that are selected for functional differences as well as the visual demands of the patient.Bifocals allow people with presbyopia to see clearly at distance and near without having to remove the glasses, which would be required with single vision correction.Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third area (with intermediate focus correction) in the middle.