Why centring matters
A lens only performs when its optical centre sits on your visual axis. A few millimetres out introduces unwanted prismatic effects that tire the eyes, and on a progressive lens it narrows the usable fields of view.
How we do it
We measure interpupillary distance, fitting height, pantoscopic tilt and frame wrap while you wear the frame in a natural position. Those figures go straight to the laboratory, which surfaces the lens to them.
We do this on every pair of glasses, not only on progressives.