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The specific boundary or zone in a multifocal spectacle lens (like a bifocal, trifocal, or progressive lens) where the optical power begins to change from the distance prescription to the near prescription.
read more about transition point ...In traditional bifocals, the transition point is a visible line separating the distance segment from the near segment (a sharp jump in power).
In Progressive Addition Lenses (PALs), the transition point is actually a transition zone (the corridor), where the power gradually and smoothly increases from the top to the bottom of the lens.
Accurate placement of the transition point is crucial for comfortable vision, as incorrect placement can cause the wearer to adopt awkward head and neck postures to find the correct optical zone.
A sharp transition point, as in a lined bifocal, causes a perceived "image jump" as the eye crosses the boundary due to the sudden introduction of prism.
The distance reference point (DRP) is usually measured through the pupil center in the primary gaze, well above the transition point.
In PALs, the power is created by complex curvature changes, making the transition point a gradual, mathematically continuous zone, hence it is invisible.