Med-Psych.com

Pupil Meter for iPhone - an easy tool for measuring pupillary distance.
You've left your eye Doctor with your prescription in hand and you want to order glasses from your favourite online store.
If you go back to your Doctor for this measurement, they will probably charge you for it. Now with Pupil Meter, you can now take your p.d. measurement with confidence.
Measurements for NEAR p.d. (12" focal point) and FAR p.d.(20' focal point) are displayed - usually the FAR p.d.is needed for online purchases.
Email your both pd’s, with attached photo of your measurements directly to your online store. Attach your name, prescription, phone number to the email for verification purposes

Pupil Meter for iPad - for greater accuracy - useful for the professional optometrist.
Works from library images taken on high end cameras, Readings can be very finely tuned - measures near and far pd as well as frame and bridge sizes.

reference: http://www.sunmoresystems.com/Sunmore_S ... Meter.html

I'm usually not such a huge fan of people promoting websites or products in these forums, but I do think that this app is an interesting one and something useful to people who visit here. The thing that disappoints me is that it isn't a free app - or at least a lot less than $5. Also, looking at this listing on itunes there is a review by a licensed optician which says that the measurements are not very accurate with this app - sometimes off by as much as 3mm. I'm not sure how much faith to put in that review, though, as the review also seems very negative towards ordering glasses online, which is the situation for which this app is most likely to be used.

Perhaps someone else can compare the accuracy of this app with real tools and also compare to the freely available methods people can do on their own.

There really is no need for an app like this. I was able to do it myself for my order for Zenni optical by using a piece of paper in front of the mirror and just marking the paper where the center of my pupils was. I then measured the distance between the marks. I think it was an accurate number because my glasses from Zenni seem very good.

I don't have an iPhone but I decided to test measuring PD in a photo based on the measurement of a known object. I took several high resolution, sharp focus photos of myself and a ruler. I made measurements using Photoshop's Analysis tools. To my disappointment, my measurement was consistently 3-4 mm less than my actual (professionally measured) PD. I suspect the reason for the discrepancy is that I was unable to hold my face and the ruler perfectly parallel. If I could do that, I suspect my measurements would be much more accurate.

I tried to measure my distance PD by myself in a mirror with the same ruler. I was not successful because I could not focus on a far-away object and look at the ruler at the same time.

I asked my wife to measure my PD with the ruler. To my astonishment, she measured my binocular PD correctly on the first try, down to the half millimeter! However, she wasn't able to correctly measure my monocular PD. (My eyes are each a different distance from centre.)

The first method you tried for measuring pupillary distance using Photoshop's analysis tool was pretty clever and seems similar to the approach used in the iPhone app. Based on your experience, I'm not sure I'd trust the iphone app method much.

I did find out that the method I used of looking in a mirror and holding a notecard in front of my face and marking my pupils worked well. The optician's measurement was only 1mm different from what I got doing it myself. Having someone else measure is probably the best option if you don't have the numbers from a professional.