That does sound like a P.D. issue.
P.D. is not specific to the frame - it's just the distance in
millimeters between your two pupils when looking straight ahead.
O.C. (optical center) is frame specific - this is a measurement of
where you pupil is centered on the lens. The P.D. determines where it
will fall horizontally, but the vertical measurement is taken from the
bottom of the frame to the center of your pupil, so this is entirely
dependent on the vertical depth ("B" measurement) of the frame.
Vertically setting the O.C. isn't that important for average
prescriptions (say, less than -5.00 diopters). It will default to
center of the lens, and even if it's off a little, people naturally
adjust to wearing their glasses further up or down their nose. You can
also adjust the nose pads in metal frames to change how the glasses
sit vertically on your face.
In higher prescriptions, and especially in progressives and bifocals,
the O.C. is more important. For higher prescriptions, because there is
significant distortion the further away you move from the focal point
(because of the high curve), the O.C. needs to be considered more
carefully. In progressives and bifocals, the location of the O.C.
determines how large the reading area will be, and in progressives,
also the intermediate area.
Online retailers estimate the seg height, or vertical measurement of
the O.C. For progressives and bifocals, there needs to be additional
consideration of this measurement. I know we follow up an order for
progressives or bifocals by asking where people generally wear their
glasses (high on their nose, middle of their nose, or lower on their
nose) to help make a better estimate. EyeBuyDirect lets you choose
between a 70% / 30% or 60% / 40% split for distance versus reading,
which is another rough way of estimating the seg height. Not sure
about the others. But, this is one area where online is not quite as
good as going to an optician, where they can take a precise seg height
measurement with the frame you've chosen.
All of that said, you can probably go to a local optician and offer to
pay them to measure your P.D. Or you could take a pair of glasses you
ordered online in and ask them to verify the prescription, again for a
fee, which would include checking the P.D. They might be willing to do
it for free, but understand they're providing a professional service
which they had to go to school for, so I think it's only reasonable
they be compensated.
I hope this helps!
Marc
JustEyewear.com
On Feb 10, 1:03 pm, da <[email protected]> wrote:
> I've ordered 2 sets of glasses from eyebuydirect. Both sets have had
> the same issue in that when I look through them, while everything is
> in focus, it feels like my eyes are being pointed in different angles.
>
> My hunch has been that it's been a P.D. issue. I've measured them
> myself in several of the methods and keep getting the same results.
>
> So, today I happened to be in Target and stopped by the optical center
> to see if they could do it for me.
>
> They told me that they could, but PD and Optical Center measurements
> are dependent on specific frames and lenses. True? BS?
>
> If BS, does anyone know what optical chains/centers will measure my
> PD? I'm fine paying for that service.
>
> -DA
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