Re: Polycarbonate versus High Index Plastic (1.67) lens help!

On Tue, 04 Sep 2007 10:09:02 -0700, KC <kebuchan@xxxxxxxxx> wrote:

Hi all,

I just picked up a new pair of glasses from Lenscrafters. They
recommended Polycarbonate because it was apparently shatterproof etc
etc. My prescription is -5 and -5.25, so I think that's fairly high.
The frames are a lot smaller, almost a 40% smaller than my old (13 yr
old) frames. Here are some of the problems I'm seeing:

1. Keeping my head facing forward if I try to look at something
slighty to the right or left of me, it's blurry.
- eg. I can see my computer monitor fine. If look at the phone on
my desk, a foot away from the monitor, I can't read the brand name or
the function buttons, they're all blurry

2. Reading stuff on my desk, all the print (black) has a yellow
outline/halo. (Chromatic Aberration?)

Also for #1, when driving if I glance at my driver's side mirror
(without turning my head), I can't read the license plate info or for
example the word "TAXI" on a taxicab's hood.

Both (blur and color) are due to Transverse (or lateral) Chromatic Aberration
(TCA). The solution is to use a less dispersive material. Look for lens
materials with an Abbe value of at least 36, but in your case, probably because
you were wearing cr39 plastic, no lower than 42. The lenses still need to be
positioned accurately regardless of the lens design.

The Lenscrafters person suggested I get "High Index Plastic", which is
a $100 upgrade. I don't want to get those and then find that they are
the same as the polycarbonate.

Depends on the Abbe of the material. 1.60 is 42, 1.67 is 32, 1.70 is 36, 1.74 is
33. Also consider Trivex (1.53, Abbe about 44), and if you opt for 1.60, I would
use Seiko's 1.60 aspheric, due to this lenses lower specific gravity (compared
to other 1.60 materials).

Does anyone know if these aberrations that I'm seeing is normal for
Polycarbonate?

In some applications, yes. That's also true for other lens materials.

And would high index plastic lenses of 1.67 make a
huge difference? What about 1.74?

Not a huge difference. I'd error on the side of better optics.

Although, Lenscrafters doesn't
carry any lens of 1.74. I think Nikon makes a lens like that but
they don't carry Nikon lenses.

Any advice would be greatly appreciated!

The frame selected may also be a factor in the lens choice. Most lenses look
thicker in rimless frames, drilled lenses require materials with greater tensile
strength. I prefer lightweight glasses; they are much more comfortable, stay in
position better, keeping the optics aligned as intended, improving
functionality. Thickness is primarily a cosmetic concern. Some thicker lenses
can be as lightweight, and sometimes lower in weight, than thin lenses.

Discuss your needs with the optician, and tell them to take off their sales hat
and put on their optician hat.

Thanks,

Kevin

Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical
Wauwatosa Wi.
~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman
.