Also, interestingly, in Mexico PD is part of the prescription.
CR39 has lower index of refraction (1.498 vs 1.54) and so it is heavier. It has worse chromatic aberration (Abbe 58 vs Trivex 45). So it's really only good for light prescriptions.
Trivex is also easily drillable making rimless easy, rimless->lighter; PC tends to crack easily.
But yes Trivex is a little more expensive than CR39 but in my experience a lot better. If your lenses last 2 years, lenses are not where you should save money.
What I couldn't figure out is whether a CR39 lens' comparative thickness (prism length) vs a thinner Trivex lens degrades its chromatic aberration advantage in practice.
http://eclecticeyewearaustin.com/wp-content/uploads/2016/05/...
However, due to the selling point of Trivex being "shatter proof" it has a greater centre thickness (to pass safety lens regulations), and thus the edge thickness is also comparable to CR-39.
It wasn't on mine either, but on advice given to me I called up the optometrist to ask and they gave it to me with no problem at all.
I've started keeping//tracking my prescription some time ago just for good measure.
Fortunately I was able to find it on a prescription from a previous provider.
However, where the optical centre of a lens is placed within a frame can vary from the PD a fair bit, depending on how the frames sit on your face, as the contact point of the frames do not always line up perfectly with the top of the nose bridge.
Also, an oft looked over measurement is the distance from the bottom of the frame to the centre of the lens, which is important with complex prescriptions and cannot be measured without looking at the frame on your face.
Having said this, with most low prescriptions the human tolerance is so high that it does not matter.
I guess I was lucky to some extent, and I hope we're all able to find "better" providers.
My eye doctor (don't remember which one he is, sorry) recommended bifocals that are computer-range above, reading below.
Simply fantastic. Couldn't recommend it more.
I have given up on progressives altogether. Single vision for me is awesome. But my big recommendation is to educate yourself on the choices available cuz you can't expect an optometrist/optician to do this in five minutes.
My PD is L 32.5 R 31.5. The ruler technique is neither this accurate nor does it distinguish left from right. Yeah, I'm a little lopsided.
I keep this information now in a Google Doc along with my eye size, bridge size, temple length and current frames. Since I'm a nerd, I read up on lens materials and now I use Trivex single vision aspherics for readers and drivers. I'm a lot more knowledgable now about my available choices. These are some of the Trivex single vision choices:
http://www.eyeglasslensdirect.com/Trivex-Single-Vision-Lense...
If you have major astigmatism problems you really must educate yourself on the choices. There are free form lenses with much more complicated computer driven prescriptions+lenses customized to the individual. Also, you can get free form lenses tailored to computer use.
http://seikoeyewear.com/lenses/progressive-lenses/seiko-indi...
http://seikoeyewear.com/news/seiko-pcwide
Another thing is that if you ask your optometrist for a prescription for readers, they'll usually set the focal distance to something like a foot. Well, I'm typing on my laptop and it's about 3 feet away. This is intermediate or tool distance and a prescription for that is a massive quality of life improvement.
If you spend some time on an online site like www.framesdirect.com, you can educate yourself quite a bit on the available choices. When you walk into an optician's office, they really only have a certain amount of time to work with you since they have to make a nickel; so you usually spend a lot of time on frames. I haven't even ordered online yet but educating yourself before you walk into a brick and mortar optician's office will make you a little annoying but then you'll get a better result.