Which index should I spring for 1.61 or 1.67?

[angerme]

I have a prescription of -4.00 should I get the 1.61 index or go all
out for the 1.67?

would one even notice the difference?

[J. Evan]

I think most everyone will tell you that there is no exact formula for
deciding this. That said, it will usually depend on whether if you
have full rim frames or rimless frames - full rim or half rim frames
will hide most or all of your lens edges very nicely, which in that
case, you likely would be fine with even with regular CR-39 (1.498
index) lenses. In the case of rimless frames, CR-39 plastic is too
soft for a drill mount application, so you would have to choose a
polycarbonate or high-index MR-7 or MR-10, 1.60 or 1.67 lenses.

Your prescription is very mildly common, so unless you are extremely
concerned with edge thickness or eye minification, then you will be
just fine with nothing higher than 1.61. You will see very little
difference in thickness between 1.61 and 1.67 in your correction
unless you've got very large lenses (above 55mm)

J.Evan

[Chuck Knight]

What is your intent for the higher index lenses...thinness, or optical quality?

If your interest is in the optical qualities of the lenses, then you
need to make sure of which higher index materials are being used, and
their respective ABBE values.

If, however, your interest is in thickness, then you need to be aware
that your prescription is not excessively strong, so the difference in
thickness for the various materials should not be overly pronounced.
Let me get you the link to the online thickness calculator:
http://www.opticampus.com/tools/thickness.php

That will show you what your lens thicknesses will be, with various
index materials. Only you can make the final decision as to whether
the additional 0.1mm of difference is worth the cost.

-- Chuck Knight

[Paul]

-4.00 is not that bad. Personally, except for rimless, I would stick
with CR39 because of low price, reasonably good thickness, and
excellent optical quality.

If you're getting plastic frames, CR39 is a complete no-brainer. The
frame will hide any lens thickness, and there's no chance of chromatic
aberration. CR39 is probably not suitable for rimless for other
reasons than thickness. I would think that it's fine also for metal
frames unless you're getting really large ones (50+ mm).

Zenni's standard 1.57 "Thin and Light" plastic" (probably the same
stuff that other places call by that name) should also work fine with
any frame.

[clarity]

I would NEVER use cr39 on a RX over +3.00 or -3.00 unless i HAD TO.
It looks like crap, sorry but like pure crap.It will stick out up to.
6.7mm i would consider this to be heavy and ugly.Sorry but just
because you want to save money doesnt mean you should buy the cheapest
product available.
Sorry all lenses have abberations, cr39 in one of the most stable
materials in this regard.However, unless you have low vision, or are
unable to be corrected to 6/6 with spectacles you are very unlikley to
notice the difference in abberations if you go for a high index.

[Ira Mitchell]

Wrong.

My prescription is -4.25 and I prefer CR39 to the alternatives. I was
(OVER) sold on high-index for years. With today's smaller frames, I
can certainly buy CR39. For a pair of aviator sunglasses? Sure, those
lenses are huge and as they flare out further from the optical center,
you're going to get some serious thickness.

I can sit all day working at my laptop without eye fatigue with CR39
in my frames. The polycarb and other hi-index shorten that time
considerably.

For a script in the -6 range, even -5 I think your point is valid,
it's just that the retailers have kept dropping that number to pad
their pockets.

Much of this boils down to vanity -- and even at -4.25 these frames
all look great. If you're concerned about thinkness, pick a plastic
frame. They'll "hide" a lot of thickness.

At my prescription, there are no physical side-effects from weight of
CR39. The skin on my nose doesn't suffer or break down from it.

- Ira

[Paul]

I have one pair of metal frames with a 38mm lens width, and with a
-6.25 prescription, the CR39 lenses stick out at most only about 1.5mm
from the frame. The maximum total lens thickness is just over 4mm.
Besides the small lens, the thickness was also minimized by having the
optical center very close to the physical center of the lens.

On another metal pair with CR39 lenses, with the optical center of the
lens closer to the nose (which is very typical), and a higher -7.00
prescription and a 48mm lens width, the maximum thickness is about
7mm, with just under 4mm sticking out from the frame. The edges aren't
polished, so I would consider this about a worst-case scenario. It's
not that good, but it's not terrible, either. I get almost exactly the
same measurements on a frame with a 50mm lens and the same
prescription.

I see three main factors - lens width, location of lens center, and
edge polishing.

The fourth factor is lens material, but so far I was only looking at
CR39 lenses. On a metal frame with a 41mm "Thin and Light" 1.57 lens
and a -6.50 prescription, the maximum thickness is 4mm, and the lens
sticks out almost 2mm. The optical center of the lens is somewhat
toward the nose, and would be better centered if not for the wide 26mm
bridge on this frame.

[John Szlachta]

It's wrong to consider +3.00 & -3.00 the same. Plus corrections are
almost doubly inverse of minus corrections - 1.5x at a very minimum. A
+3.00 is at least a -5.00 when comparing severity. Just ask any lab as
to which stock lenses they carry, +4.00/-7.00 to -8.00 is probably
most common.

Minus corrections are more concerned with edge thickness. Plus lenses
can get very heavy. I've seen virtually no one happy with a +3.00 or
above in cr-39, or +3.50 in polycarbonate or above. I've dealt with
almost 10 optical professionals in 3 states & 2 countries in the past
ten years. This is the most common mistake i've seen made by
inexperienced personnel.

J.evan

iPhone

[Paul]

We pretty much were only discussing minus prescriptions.

With plus prescriptions, isn't it getting common to reduce thickness
with aspheric lenses? I even see aspheric lenses on $10 drugstore
reading glasses.

[John Szlachta]

You're right that plus prescriptions benefit much more from aspheric
design than simply a decrease in thickness. All the 1.67 lenses i've
ever seen were aspheric as well, although the designs vary, perhaps
with Seiko 1.67 being the gold standard.

iPhone

[clarity]

Well

> > > any frame.- Hide quoted text -
>
> - Show quoted text -

[Paul]

Ira, have you tried the "Thin and Light" material that Zenni uses in
their standard lenses? I'm asking because I've had glasses in the
distant past that I had to return because of chromatic aberration (and
I know that at least one of them was supposed to be "real"
polycarbonate), but the Zenni lenses seem fine to me, with very little
noticeable aberration on only some of the pairs I've gotten.