Surfing the net, I ran into a site selling relatively cheap prescription eyeglasses (free plug: Zenni Optical, frames as cheap as $8.95, I got the 3148 Rimless Flexible (Memory) Titanium for about $50 with all options). I was able to locate my 2 year old prescription, which I know is still good because I see perfectly with the scratched pair of glasses I am replacing. I have astigmatism, which doesn’t change very often, and my prescription is a rather minimal -0.50 SPH in both eyes.
I started to input my prescription into the site, and it seemed that I had everything required to order prescription eyeglasses over the Internet! But just as I was getting ready to hit “order,” I saw this: “PD-Pupillary Distance — MUST SELECT.” I said to myself, okay, it must be on my prescription somewhere. I looked and looked and I could not find it anywhere. So I clicked the question mark, which brought up the following (emphasis mine):
The PD, or Pupillary Distance, is the distance between your two eyes, center to center, the center of one pupil to the center of the other, in Millimeters (1” = 25.4mm). Unfortunately most optometrists leave this quite necessary measurement off from your prescription, but you can measure this yourself.
You can measure this yourself in a mirror (not very accurate, and not generally recommended) or have a friend measure it for you by holding a ruler just under the pupils of your eyes and measuring the distance center to center. You should be looking straight forward fixing your focus at distant object, and not at the person measuring you.
Average is about 62mm, and most, 97% or so, fall within 54 to 74 range. If you are coming up with a number outside of this range consider that you might be making a measurement mistake. Do NOT just guess, or assume you’re just an average 62, and DO NOT measure your old glasses to try to come with this number.
Fair enough. I located a ruler and started measuring my PD looking in the bathroom mirror. I am pretty thorough as I think that any task worth doing is worth doing right, and after a few trials I ended up with something like 65mm.
But.. my scientific mind wondered if there was a more precise way to go about doing this. After all, these are my eyes we’re talking about. There had to be a better way! My Canon 400D digital SLR came to mind, which I set on my Manfrotto tripod, taking about 5 shots and ending up with this:
In case you’re wondering why the metric end of the scale is upside-down: the glare from the flash was making it unreadable the other way around, so I flipped it to make the US end of it (inches) unreadable instead. This is perfectly acceptable and does not affect the accuracy of this method. You should be able to do this with any digicam, just make sure the ruler is aligned straight with your eyes!
Note: this method should only be used to measure your distance vision PD, not your near-vision PD (i.e. reading glasses). I am not an optometrist, and all information presented above is purely for informational purposes — use at your own risk, if in doubt go to a professional!