How accurate must PD be? Eyeglasses
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From: me on 25 Jul 2010 19:06 I've had my PD (distance between pupils) measured three times at Costco so can order glasses online Each time it has been one half to one complete mm off. How important is PD and how accurate must it be? I'm thinking I should take all three measurement and average them and use that.... one for the right side and another of the left side. Good idea?
From: Rod Speed on 25 Jul 2010 19:11 me(a)privacy.net wrote: > I've had my PD (distance between pupils) measured > three times at Costco so can order glasses online > Each time it has been one half to one complete mm off. Thats nothing much. > How important is PD and how accurate must it be? Not very, the use of over the counter glasses proves that its not that critical. Its just the distance between the eyes, and clearly with simple prescriptions, the distance isnt that critical. > I'm thinking I should take all three measurement and > average them and use that.... one for the right side > and another of the left side. Good idea? Cant do any harm to use that approach.
From: bart.c on 25 Jul 2010 19:30 <me(a)privacy.net> wrote in message news:6qgp46dtsi6t4jldqhvn1982mkj2ekkqa4(a)4ax.com... > I've had my PD (distance between pupils) measured three > times at Costco so can order glasses online > > Each time it has been one half to one complete mm off. > > How important is PD and how accurate must it be? > > I'm thinking I should take all three measurement and > average them and use that.... one for the right side > and another of the left side. Good idea? Mine has been measured at 62 to 65mm at various times. I don't think it's critical, as this distance varies anyway (when you look at closer objects, the pupils come together), and your eyes will make minor adjustments themselves, within reason. (But if you want to double-check it yourself, use a ruler and a mirror: look at the right-hand eye in the mirror, hold the ruler so the 0-mark is aligned at the centre of the pupil; now look at the other eye in the mirror, and read off the distance on the ruler of the centre of that pupil. Now you have your PD for when both eyes are looking straight ahead. Another way is using an existing pair of specs: draw a vertical line on a sheet of paper. Hold one lens over the line so that the image through the lens is centred (line is not refracted left or right). Mark lens with centre of line with a dot. Repeat for other lens, and measure the distance between dots. But note that with reading (close-up) specs, the lens centres may be nearer than your PD for distance) -- Bartc
From: John Weiss on 25 Jul 2010 19:32 me(a)privacy.net wrote: > I've had my PD (distance between pupils) measured three > times at Costco so can order glasses online > > Each time it has been one half to one complete mm off. > > How important is PD and how accurate must it be? > > I'm thinking I should take all three measurement and > average them and use that.... one for the right side > and another of the left side. Good idea? Depends on the glasses... With bifocals or trifocals with narrow field of view in the inset lenses, it will determine how well you can see at close ranges. With progressive lenses, many of which have an EXTREMELY narrow field of view in the mid- and/or close-range fields, it can be critical to eye comfort. My wife was never able to adjust to the progressive lenses she got from Costco, even though she got the measurements in the store, not online. With proper measurements and good lenses from another optician, she adjusted rapidly.
From: Salmon Egg on 25 Jul 2010 20:21
In article <8b3ukbFq85U1(a)mid.individual.net>, "Rod Speed" <rod.speed.aaa(a)gmail.com> wrote: > me(a)privacy.net wrote: > > > I've had my PD (distance between pupils) measured > > three times at Costco so can order glasses online > > > Each time it has been one half to one complete mm off. > > Thats nothing much. > > > How important is PD and how accurate must it be? > > Not very, the use of over the counter glasses proves that its not that > critical. > > Its just the distance between the eyes, and clearly > with simple prescriptions, the distance isnt that critical. > > > I'm thinking I should take all three measurement and > > average them and use that.... one for the right side > > and another of the left side. Good idea? > > Cant do any harm to use that approach. I measured that distance by holding a metric tape (not strictly necessary) under my eyes and having my picture taken. It was no problem reading the scale to about a millimeter. Even if there is an error, it is equivalent to having a bit of prism added. It is a relatively simple thing to calculate, but it dependent upon the spherical correction. I chose to have my error on the side of additional convergence because one ey points out. Bill -- An old man would be better off never having been born. |