PD measurements from my two exams.

[James]

Recently, I have my eyes checked and got a new prescription. It was
not much different from the one I did in Sep 2005. One thing I noticed
is the PD. My old pers is 71mm and the new one is 63 for reading and
67 for distance. If I want to order a pair of glasses online, which
value I should use?
Thanks.

[KayakerNC]

If both prescriptions were from the same place, you might want to ask
them why the big (6mm) difference. Normally and adult's PD will stay
pretty much the same, since your head is no longer growing.
The distance PD is the value to use when ordering online. Normally
the near PD is 3mm less than the distance, as is your measurements.
Or try WalMart for their machine measurement.

Kayaker

[powrwrap]

> On Jan 4, 1:14 pm, KayakerNC <mystery1234...@gmail.com> wrote:
> If both prescriptions were from the same place, you might want to ask
> them why the big (6mm) difference.   Normally and adult's PD will stay
> pretty much the same, since your head is no longer growing.
> The distance PD is the value to use when ordering online.   Normally
> the near PD is 3mm less than the distance, as is your measurements.
> Or try WalMart for their machine measurement.

Actually the difference between reading and distance pd is 4mm. I
would question that. However, the bigger question is how your pd could
have "shrunk" from 71mm to 67mm?

I'd get my pd re-checked at a different location. Or try it yourself.

[James]

Thank you. The two prescriptions are from two diff offices. Will give
WalMart a try.

> > Thanks.- Hide quoted text -
>
> - Show quoted text -

[James]

I had glasses made from each of the optometrist after each exam and
they both seems to be OK. Can both readings be "OK" because I selected
large glasses (50mm wide by 36 mm high) and both measurements ended up
in the middle of the lens?

[powrwrap]

No, both readings cannot be correct. The distance between the center
of your pupils is constant!

However, the fact that you had large lenses may have allowed your eyes
to more easily adapt to the discrepancy.

[jaho...@googlemail.com]

James, I would expect the difference between reading and distance PD
to be nearer 2mm than 4mm. The difference occurs because your eyes
swivel "inwards" to see an object close to you (an extreme example is
when you focus on he end of your nose!). But as Kayaker says, your PD
(the distance between the center of your pupils) is constant once your
head stops growing.

With prescriptions with a power less than 3 a discrepancy of up to 2mm
can usually be tolerated. The ability of our eye to compensate for
discrepancy is what the online retailers who don't insist on a PD rely
upon when they set all their PDs at the average of 62mm. For powers
greater than 3 and especially with progressive lenses absolutely
accurate PD measurements are required for optimal results. Accurate
PDs are measured using a device known as a pupilometer, all opticians
will have one so I would suggest that you track one down that is
prepared to provide you with values for distance and reading PDs.

Cheers,
James

[James]

I went to two Costco stores today and got couple of PD readings:

Costco 1 - 68 (34.5 + 33.5) for distance. Did not do any measurement
for reading.

Costco 2 - 69 (35 + 34) for distance. 64 (32 +32) for reading.

[powrwrap]

How did they take the measurements? Did you look through one of those
binocular devices or did they measure it by using a metric ruler?

As (another) James has said, that 4mm difference between vision and
reading seems like a lot.

[James]

Yes, all the Costco's use a binocular like device.

I went to another one today and my distance reading is 68 (35 +33).
They refused to do measurement for near.
Thanks.

[jaho...@googlemail.com]

Hi James,

The 4 mm difference between your distance and reading PD is entirely
normal, so I would go with the figures provided by "Costco 2". The 1mm
difference between the distance PD found by different opticians using
a pupilometer is acceptable. My guess is that the 71mm PD from your
old prescription was measured using a ruler, it is difficult to get a
accuracy of greater than +/-2mm with a ruler so that could explain the
variation.

If you are buying general use single focus lenses then supply the
distance PD, if you buying glasses for distance use then give the
larger PD and for reading the smaller PD. If you are going to buy
progressive (varifocal) lenses then you'll need a supplier that will
take account of both measurements.

Hope all of this help!

Regards,
James

[KayakerNC]

Hey James,
With the machines, a 1 mm difference is TINY, and is probably within
the tolerances of the pupilometers. Majority opinion seems to be
68mm, you can probably order online with that, and be satisfied.
I hope you tried on some of the glasses frames at Costco, and wrote
down the numbers from the frames that looked good AND fit well. You
could also carry a small mm ruler to check the lens height on frames
that you like.
Good luck,
Kayaker

[James]

Thank you for everyone's replied. I have place an order with
Optical4less using 68 mm as my PD. I managed to find a frame with
similar dimension as my existing one and hope all will work out.