Progressive lenses need much more accuracy to be fitted correctly. This means a mono-PD (separate PD for each eye, not just the distance PD halved) and also a height, usually from "datum" measured for each eye too. This measurement would be different depending on the frame and how it sits/adjusted to your face.
This means the frame must be chosen and fitted to you before any of these are measured.
It becomes a 2D measurement.
There is a lot more to it (vertex distance, pantoscopic tilt etc) but the above is the absolute minimum.
This means the frame must be chosen and fitted to you before any of these are measured.
It becomes a 2D measurement.
There is a lot more to it (vertex distance, pantoscopic tilt etc) but the above is the absolute minimum.