PD for Progressive Lenses, Segment Height and Fitting Height
For bifocals the extra figure is segment height, and for progressives it is fitting height; your PD alone is not enough. Your chosen frame affects each one.
- 18 to 22 mmlowest fitting height most standard progressives need
- 8 to 12°typical pantoscopic tilt
- 14 mmvertex distance to use if none is listed
PD Measurement for Progressive Lenses
For progressives, your distance PD works best as a dual PD so each reading zone lines up with its eye. For simple single vision lenses, a measurement taken at home is enough. With progressives, an optician takes the measurement while you wear the frame, and that result is the one to rely on (Cleveland Clinic).
Segment Height and Fitting Height
On lined bifocals, segment height (seg height) runs from the lens bottom to the top of the reading area. Progressives use fitting height, or OC height, measured from the lens bottom to your pupil centre. For most standard progressive designs, the fitting height has to be 18 to 22 mm (Optogrid); a total lens height of 28 mm or more also helps (Zenottic).
How to Measure Segment Height
With the new frame on, look into a mirror and have someone mark each pupil centre on the demo lens while you look straight ahead. From that mark, measure down to the inside lower edge of the frame. Adjust the frame first to the position you will wear it in, then measure.
Pantoscopic Tilt
Pantoscopic tilt is the inward lean of the frame front at its lower edge. A range of 8 to 12 degrees is usual (Lens.com). For every 2 degrees of tilt, the optical centre moves down 1 mm, as a general guide (National Academy of Opticianry).
Vertex Distance
Vertex distance is the space from the back surface of the lens to the front of your eye. If the doctor does not state one, labs work with 14 mm (National Academy of Opticianry). It has a bigger effect when the prescription is strong.
Questions About Progressive Lenses
Which PD should I use for bifocal lenses?
Use the distance PD for this. The lab figures out the near PD from that and uses it to position the reading segments.
Does OC height mean the same thing as fitting height?
They are two names for the same thing: how high the pupil centre sits from the lens bottom.