What does "add" refer to in a bifocal prescription?

In a bifocal prescription, "add" specifically refers to the additional power required for near vision correction. This term indicates how much extra magnifying power is needed for close-up tasks, such as reading or sewing, beyond the distance prescription.

In bifocal lenses, there are typically two zones: one for distance vision and one for near vision. The "add" power is always a positive value that represents the difference in diopters between the two areas of the lens. For example, if a patient has a distance prescription of -2.00 D and an add of +2.00 D, the near vision portion of the lens would provide a total power of 0.00 D for tasks up close.

This distinction is crucial for understanding how bifocal lenses accommodate visual needs at varying distances, making the "add" a key element in lens design for individuals with presbyopia or other vision changes requiring multifocal correction.