I've been with my optician quite some time now and my optometrist has gone to great measures and used various techniques to calculate my PD. So why is it that sometimes when I order new lenses they want to measure my PD again? Surely the distance between my eyes is fixed ... unless my face expands or contracts over time.
We often take PDs from customers even though we have the information in the file already. Sometimes we just like to double/triple/quadruple check everything, sometimes, there can be an error between what was keypunched into the computer and what we wrote by hand, and we just want to be sure.
There's no such thing as taking too many measurements, nor having too much information.
:cheers:
I'm concerned because my optomotrist has measured my PD several times using different techniques and come up with a figure ... and today one of the employees that deals with ordering/fitting, etc. came up with a different figure - about 1.5mm different to that of the optometrist ... when I questioned the difference she thought I was being awkward.
So at the grand ole age of 35 is it possible that my PD has physically changed 1.5 mm over the course of about a year?
Something changed.;)Originally Posted by mirage2k2
1.5mm or 0.75 per side. This is a difference, however unless your prescription is very very strong you would not detect much difference.
You may also want to check and see if one measurement is a Far PD and the other is a near PD since the range can be for 2 to 3mm smaller total for near than far.
You can only obtain a limited amount of accuracy with such subjective measuring techniques.
Did you move one eye slightly before the optican saw the 2nd eye?
todays PD was measured using those space-age looking binocular thingos ... you look in and stare at the green dot ... these really dont work well for me because I see 2 dots ... its very funny ...
they say "please focus on the green dot",
I say "which one?",
they say "there is only one",
I say "no, I see 2"
:D :D :D
... at this point they decide to do one eye at a time.
Anyway, the only reason I care at all is because my optomotrist and I are in the middle of ironing out some complicated prescribed prism issues (amounts and distribution) ... and the LAST thing we need right now is for somebody to take an incorrect PD measurement and then I end up with new lenses that contain a load of induced prism because they are accidentaly decentered.
Why cant they just stick with the same PD as before ... why change more than one thing? If the new lenses are good, or bad, we will not know whether the difference was due to the change in prism or the change in PD
I should ring them and talk to the optomotrist himself and ask him if this 1.5mm difference really matters ... but I already feel like I have become a pain in the backside :(
If you have been wearing glasses that are off by 1.5mm in PD and you keep the old PD measurement you likely feel better wearing the new prescription, since your brain has compensated for this slight difference.
The brain is involved in processing all of this it is not just optics, and to confuse you a little further, you could go to five different doctors for a prescription and I will almost guarantee you that all five will not be the same. In fact you will probably have at least three different prescriptions for vision. They will be similar but not the same.
Each doctor may have their own technique for example.
How strong is your prescription, you maybe worrying about something that does not matter.
+6, 5 base out prism, PD 66.0 - 67.5mm
I'm not sure if 1.5mm would induce much prism in +6 - I should look at the Prentice's rule thread ... but the presence of prescribed prism leaves me confused about how to apply the rule.
I'm still very interested to know, however, if a persons physical PD can change as they age?
I think I should ring them in the morning just to put my mind at rest. If they think I'm a pain in the backside then so be it.
Your presciption is strong enough to need more precision, and being a + it is more critical.
As far as you eyes PD physically changing that much in one or two year, I would doubt that is possible.
Measuring PDs generates different answers by different people doing the measuring.
Agreed.
...and if the doctor is trying to calculate how much prism to add, if they use the old PD, they won't have an accurate measure of how much prism you are actually looking through.
The presence of prescribed prism has no effect when applying prentice's rule.
thanks for all your help guys ... I think I'll get them to double check with the optometrist ... I've just calculated that with my rx (+6d) this 1.5mm difference will equate to a difference of almost 1d of prism ... this is quite significant ... all we are trying to achieve in the new lenses is a different distribution of the same amount of prism (5d). If the PD in the new lenses is 1.5mm less then I'll effectively end up with about 4d of prism... we've already tried 4d and it is not enough!
Actually, not too far out an idea. I think that when people put on a lot of weight, the p.d. can increase from increased orbital fat pushing the eyes outwards. Likewise, losing a lot of weight can reduce orbital fat, causing the eyes to receed somewhat, making the p.d. smaller. Also, exophthalmos caused by graves disease and other conditions can definitely increase the p.d. to increase. Finally, a changing amount of fluid in the orbital tissues, say from edema, could do it.
The reason the p.d. can change from these things is that the orbits are roughly pyramidal in shape, with their axes converging toward the back of the skull.
But normally, unless I'm suspicious of one of the above effects, I don't measure p.d.s more than once, or unless for some reason I think I might have measured it wrong, or a recording error.
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