Re: CR39

In 1193595686.296473.184820@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
p.clarkii@xxxxxxxxx wrote:

On Oct 28, 11:53 am, Tony Houghton <h...@xxxxxxxxxxx> wrote:
Are most basic plastic lenses made of CR39, or is there another
basic plastic that's significantly thicker? I've just bought some
glasses and I think the lenses are polycarbonate because the
chromatic aberration is quite severe and they're definitely thinner
than my old glasses (even though the new frame is larger you can
still see a clear difference at the edges) which have Transitions
from Dollond & Aitchison, made by Nikon I think. The optician I
ordered them from seems to think they're CR39 though.

its hard to know for certain but polycarbonate is known for high
chromatic aberrations and it's higher refractive index does result in
the lenses being thinner. Nikon does indeed make photochromic
polycarbonate lenses and most optical shops like to use them because
they are more profitable for them so I suspect that your suspicion is
correct.

an experienced optician can listen to the sound of the lens tapping
against a hard surface and distinguish plastic versus polycarbonate.

the optician you ordered them from would know *exactly* what they are
made of unless he/she is trying to be deceptive.

You've got the wrong end of the stick a bit. My current glasses are
Nikon Transitions - "Basix" I think they're called - but they're not
polycarbonate, they're thicker plastic (CR39)? They did make them with
polycarbonate at first but I got them remade with plastic when I found I
couldn't see properly.

I wanted a second pair that aren't Transitions because in Winter I want
to take advantage of every little scrap of sun, not have it blocked out,
so I ordered some cheap ones online, expecting them to be plain plastic,
but I'm sure they're not. Funnily enough their address turned out to be
local but they say that's only a dispatch centre and I can't call in
there. That could sound suspicious, but I still think it's a mistake
rather than deception. Polycarbonate lenses aren't actually cheaper to
make than CR39, are they? Just a higher profit margin because they
usually charge more for them, which this optician hasn't done.

--
TH * http://www.realh.co.uk
.