Calculate Pupil Distance (PD) for frames?

Question

I'm looking at a bunch of frames on eBay that don't have the PD measurement; however, they do include a eye/bridge measurement. Is PD = 1/2 eye + 1/2 eye + bridge = eye + bridge?

Answer 1

It's a bit difficult to understand what you are trying to find out.

A frame that is a 50-18 for instance will be 68 mm ( 50+18) from geometric center of one lens to the other.

If your own PD...the distance between your pupils, is 64 mm, and the frame centers are at 68mm, then when the lenses are made, they will be decentered inwards 2 mm each to keep the optical centers at 64mm apart.

The smaller your PD is and the larger the frame is, the more decentration is needed.

A decentration of 4 or 5 mm each side is common, and not a concern in mild prescriptions. So your PD can be 8 or 10 mm less than the frame measurement sometimes and still not be a problem, unless your prescription is a strong one.

Some people have a wide head and close set eyes, so there is always going to be a fair amount of decentration needed.

If it is the actual size of the frame you are trying to figure out...then a 50-20..will be 50X2 + 20 = 120 mm from outer edge of one lens to outer edge of the other lens.

So there is no such thing as a recommended PD, because that is a fixed measurement on you that doesn't change.

But, certain frame sizes are recommended to keep your PD distance measurement and geometric centers of the frame as close together as possible when the prescription is strong, to reduce thickness.

Not sure if this explains what you wanted to know...

Answer 2

The pd measurement is not a frame measurement, it's a measurement of how far apart your eyes are from pupil to pupil.