FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups 
 ProfileProfile   PreferencesPreferences   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Forum index » Medicine forums » vision
Causes of chromatic aberration
Post new topic   Reply to topic Page 1 of 1 [4 Posts] View previous topic :: View next topic
Author Message
Tony Houghton
medicine forum beginner


Joined: 19 Jul 2006
Posts: 5

PostPosted: Wed Jul 19, 2006 11:40 am    Post subject: Causes of chromatic aberration Reply with quote

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?

--
The address in the Reply-To is genuine and should not be edited.
See <http://www.realh.co.uk/contact.html> for more reliable contact addresses.
Back to top
Mark A
medicine forum Guru Wannabe


Joined: 30 Apr 2005
Posts: 226

PostPosted: Wed Jul 19, 2006 6:26 pm    Post subject: Re: Causes of chromatic aberration Reply with quote

"Tony Houghton" <this.address.is.fake@realh.co.uk> wrote in message
news:slrnebsa2r.53c.this.address.is.fake@realh.co.uk...
Quote:
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?


Polycarbonate is 1.59 index, and has the worst optical quality of any
commonly prescribed lens with an abbe value of 30. Abbe value is a measure
of chromatic aberration (the higher the better). Chromatic aberration is
especially noticeable in higher power Rx's (not sure what your is).

Here are some representative lenses materials from Sola that shows the
material, index, and abbe value:

CR-39 Hard Resin 1.50 abbe 58
Spectralite 1.54 abbe 47
Polycarbonate 1.59 abbe 30
Finalite 1.60 abbe 42
1.66 Plastic 1.66 abbe 32

If you need safety glasses (like Polycarbonate) then choose Trivex instead.
It has an index of 1.53 and abbe value of about 42. Trivex is available from
Hoya Optical and Younger Optical.
Back to top
Mark A
medicine forum Guru Wannabe


Joined: 30 Apr 2005
Posts: 226

PostPosted: Wed Jul 19, 2006 6:30 pm    Post subject: Re: Causes of chromatic aberration Reply with quote

"Mark A" <nobody@nowhere.com> wrote in message
news:VaqdnVXT8vHd5CPZnZ2dnUVZ_qydnZ2d@comcast.com...
Quote:

Polycarbonate is 1.59 index, and has the worst optical quality of any
commonly prescribed lens with an abbe value of 30. Abbe value is a measure
of chromatic aberration (the higher the better). Chromatic aberration is
especially noticeable in higher power Rx's (not sure what your is).

Here are some representative lenses materials from Sola that shows the
material, index, and abbe value:

CR-39 Hard Resin 1.50 abbe 58
Spectralite 1.54 abbe 47
Polycarbonate 1.59 abbe 30
Finalite 1.60 abbe 42
1.66 Plastic 1.66 abbe 32

If you need safety glasses (like Polycarbonate) then choose Trivex
instead. It has an index of 1.53 and abbe value of about 42. Trivex is
available from Hoya Optical and Younger Optical.


I noticed that you did post your approximate Rx at -6, -7. Such a strong Rx
could worsen the low abbe value effect of polycarb.

Also, as you mentioned in your original post, a contributing factor could be
the aspheric lens, which does cause some distortion if not fit exactly
correct. The combination of polycarb and aspheric can cause the distortion
you see.
Back to top
Tony Houghton
medicine forum beginner


Joined: 19 Jul 2006
Posts: 5

PostPosted: Wed Jul 19, 2006 7:33 pm    Post subject: Re: Causes of chromatic aberration Reply with quote

In <VaqdnVXT8vHd5CPZnZ2dnUVZ_qydnZ2d@comcast.com>,
Mark A <nobody@nowhere.com> wrote:

Quote:
CR-39 Hard Resin 1.50 abbe 58
Spectralite 1.54 abbe 47
Polycarbonate 1.59 abbe 30
Finalite 1.60 abbe 42
1.66 Plastic 1.66 abbe 32

Thanks, that's quite a help. I was thinking of trying this company:
<http://www.spexmaniac.co.uk/>. They say they fit 1.6 index lenses as
standard but also say all their lenses are CR-39, so there seems to be
some discrepancy. Does this mean at best they don't really know what
they're fitting and should be avoided?

My current glasses are from Specsavers. They were able to tell me they
have a RI of 1.6 but I don't know what material they're made of. Does
anyone know what Specsavers were using as standard about 4-5 years ago?
I'd be happy to go with that type of lens again except I would like a
Transitions version.

--
The address in the Reply-To is genuine and should not be edited.
See <http://www.realh.co.uk/contact.html> for more reliable contact addresses.
Back to top
Google

Back to top
Display posts from previous:   
Post new topic   Reply to topic Page 1 of 1 [4 Posts] View previous topic :: View next topic
The time now is Wed Nov 21, 2018 8:12 am | All times are GMT
Forum index » Medicine forums » vision
Jump to:  

Similar Topics
Topic Author Forum Replies Last Post
No new posts New lenses have high chromatic aberation Ugh! GeorgeB vision 12 Tue Jan 24, 2006 11:21 pm
No new posts Spherical aberration William Stacy vision 21 Fri Jul 29, 2005 2:20 pm

Copyright © 2004-2005 DeniX Solutions SRL
Other DeniX Solutions sites: email marketing campaigns , electronics forum, Science forum, Unix/Linux blog, Unix/Linux documentation, Unix/Linux forums


Powered by phpBB © 2001, 2005 phpBB Group
[ Time: 0.0137s ][ Queries: 13 (0.0016s) ][ GZIP on - Debug on ]