I recently tried Nikon Transistions Super lenses but couldn't get on
with them because of severe chromatic aberration. They were high index
(not sure whether 1.6 or 1.7) plastic - polycarbonate I think - and
aspheric. I also noticed things looked different sizes and with
distorted aspect ratios depending on which part of the lens I looked
through. Is that a form of barrel distortion? I didn't really notice
that straight lines looked curved.

I've had a lot of trouble finding out the cause of this problem. A
definitive answer would be nice because with my prescription (about
-6/-7 in each eye depending on which whay you read the astigmatism) I
could benefit a lot from thin lenses.

Most opticians I asked seemed unaware that such a problem could exist.
One or two, including the optician that tested me last, said it's caused
by aspheric lenses. That's plausible because my old glasses are 1.6
index non-aspheric and, while I can see some chromaticity at the edges
if I look for it, it isn't a problem. However, according to
<http://www.allaboutvision.com/lenses/aspheric-lenses.htm> aspheric
lenses should improve vision if fitted correctly, and my optician and
his staff were adamant that they were a good fit.

When I discussed it on sci.optics they settled on a different
explanation: the lenses' refractive index varied too much with
wavelength because of a too low Abbe number. That's plausible too
because there's a clear cause and effect there.

Is it just that or a combination of factors? I've been lurking here a
few days and saw someone having similar problems get advice to avoid
polycarbonates. But he also said he'd been using CR-39 without problems,
and that's a type of polycarbonate. Are other, inferior, polycarbonates
commonly used in glasses?