Originally Posted by
drk
I'm going to step in it, here. I'm kind of locked into "old school" thinking, so someone will clean my mess up.
Let's talk basic, non-individualized SV: If we were to put a "fitting cross" on a SV lens (which is actually a felt-tip pen dot for most of us) we'd order the "prism reference point" 3-4 mm below it. That's because the eye looks down to read, in addition to straight-ahead, so we tilt the lens bottom in by 5-10 degrees to maintain equidistant vertex, and we have to drop the "prism reference point" (optical center) in order to have the optical axis of the lens co-incident with the eyes' ephemeral center of rotation.
So what's the "drop" in a SV? 3-4mm. Sound familiar?
So, in an old-school PAL, the same principle applied. I believe. It's "equal vertex in S.A and down gaze ---> 8 degrees pantoscopic tilt ---> 4 mm drop of the O.C./PRP."
Now I'm told in modern free-form complex surfaces that all that is blown up. Maybe I'm wrong. That jibes with her statement: "The drop of a progressive lens is not in and of itself a design element, and it is not directly related to either the progressive profile or placement of unwanted cylinder power. It is, quite simply, the lens designer’s chosen placement for the prism reference point in relation to the fitting cross"
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