I asked this, of Zenni, a long time ago. Here's what they told me.
They start with an assumption that the glasses are relatively centered
(vertically) on your face, or that they can be adjusted to be this way. The
nosepads on metal frame glasses are more forgiving than other styles, in
this respect.
For progressives, they place the corridor at 49% of the lens height, so that
it falls *just* below the middle of the lens. It's a "one size fits MOST"
approach...by adjusting the nosepads they can adjust where they sit on your
face, and can be positioned correctly, in this way.
>From memory, they position the lined bifocals at 40% of lens height.
This seems to work for a LOT of people...and is *way* simpler than the
complicated measurements most use.
-- Chuck Knight
On Fri, Feb 6, 2009 at 10:38 AM, J. Webber <[email protected]> wrote:
> I am wondering how these companies decide where to start a progressive (er,
> progressing),
> and how they determine where to put the line in a lined-bifocal. Doesn't
> this depend on the way
> a frame fits a particular face?
>
> >
>
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