How choose a base curve?
hol...@cneuro.zool.ohiou.edu
beaten to death previously. If there is an FAQ, please let me know.
How does one go about choosing a base curve for glasses? When
would one choose a base curve of 0 instead of -3 for example?
I have reached the age where my -10.50 eyes require a +1 addition
for reading, and I got glasses with Varilux lenses. With these
lenses, my peripheral vision was terrible--I couldn't look in my
rear-view mirror and see what was there without moving my head.
When I complained about this, they decided to change the base
curve from 3 (or was it -3) to 0, saying that this would match
the base curve of my old single vision lenses. I could not get
a straight answer about whether this would help my peripheral
vision or not. Each of the techs gave me a different story.
Then the O.D. came in and said that there shouldn't be any
peripheral distortion with distance vision, but only for reading.
Is this correct?
Anyhow, I am finding that with the zero base curve lenses that
there is more distortion across the width of this computer screen
than with the 3 or -3 base curve lenses, but I don't know if that
is because of the base curve change or some other factor (different
lab making the lenses). Is there any reason to pick one base
curve over another when getting glasses?
--Bill
SpecOPTICS
How we determine the "base" curve of a lens is based on the amount of power
we are dealing with in the RX.
The optimum curve for the ocular (backside) is a 6 curve..this gives you
the best non-distorted field of vision. the steeper or flatter the curve
(backside) it will have an effect on your visual field (disortion,oblique
astigmatism) .. a good way to see this is take a flat pc. of glass and look
str8t through it..nice non distorted image ..right?...now start "tilting" that
same pc. of glass and see how the image gets "distorted"...thats "oblique
astigmatism"...the same basic principles apply to your glasses...when you look
str8t through your lens you get the best visual field..but as your eyes move up
that steep curve then you get distorted vision. You try to make this less of a
"problem" by using diffrent materials (mid index,high index) or a "flatter"
front surface (base curve) to get a "flatter" back surface. Higher the minus
power (sphere) flatter the front surface, higher the plus power (sphere) more
curved the base curve.
Depending on the lens design and material we have curves from -6
(bi-concave ..looks like this )( to plano(FLAT) :-) to upto a + 21 (aphakic's)
Here is an easy example (simplified) say I get an RX of +2 sphere in my
lab then I would put them on a 7.25 base curve
7.25(base curve)-(+)2sphere (power)=(-) 5.25^ backside curve :-)....
The steeper (and flatter as well) backside curves do influence how images
are perceived by the wearer..thats simple mathmatic's...the more curved (fish
bowl look) the smaller the non distorted area of "clear" vision. One way we try
to work around the curvature problems is the use of diffrent indexes and
surface designs. These things also have their own set of problems..as you
increase in index you are able to have a less amount of curvature and still
have the same amount of corrective power BUT as you increase in index you lose
light transmision (fresnels equation) as well as have a higher degree of
chromtic problems (those nasty "rainbows") so you use surface design to help
there (spherical,aspherical,atoric) to help with the oblique astigmatism
problems that tag along with using these thinner lens designs in higher
indexes.
A good anti reflective coating helps with the light transmission problems
and enternal and external reflections.
It can get a little complicated:-) But you try to base the curvature of
base selection on the power being "ground" into the lens.Alot of our choices
are based on what is important to the patient..some are more interested in just
the cosmetics (don't ask me why) and less in optical performence. Mainly we try
to balance it all so you get the "best" possible mixture of pro's and con's..
this is why you should deal with an EXPERIENCED optical person who knows optics
and can give you the best possible combination and information to help you make
the best possible selection in lens design,lens material and frame selection
and fitting! :-) The higher or lower the power the more important these factors
become.
If after all this I missed something feel free to post away..thats why we
regulars keep coming back to this group (even if we are bashed alot :-)
JeffT
(spe...@aol.com)
Kevin and Christine Jackson
and magnification of a lens. Matching base curves is generally a good
idea but is probably not the reason you are having problems with
progressives. Progressives are designed to have a lane of clear vision
along the path the eye would normally take to read (down and in) with
increasing add power. Along the sides of this lane of clear vision is
an area of warpage. Most people are able to adapt to these lenses and
are not too disturbed by these areas, some are more sensitive to them.
In general people learn to compensate by learning to move their head
more and their eyes less. Give yourself a fair chance to adapt to the
progressives and if you continue to have problems perhaps progressives
will not be a good fit for you.
Kevin Jackson, O.D.
Carl Seutter
-10.50D is outside of the range of powers approved for progressives. To get
this power on a progressive lens is to throw out the general rules of
corrected curves. THis will naturally distort peripheral vision. If you
preferred the steeper base curve, go back to it since there is no base curve
available in a progressive that will give the vision qualities promised by
the manufacturers. Base curves are greatly misunderstood by most people who
should know better. The course "Geometric optics in ophthalmic design"
clears up the confusion for the people able to handle the math.
A -10.50 on a 0.50 base curve would need about a -11.00 back curve. Well,
this is far from the -6.00 back curve ideal. The farther you get from -6.00
back curve, the worse your peripheral vision is the general rule.
Matching base curves is an old leftover from when you had a choice of crown
glass or crown glass. It doesn't have much meaning today with over 15
different materials in both corrected curve and ashperic designs. When you
check true and indicated sagittal values, you realize that "match old base
curve" has no real meaning today. A 1 base CR-39 and a 1 base high index
1.55 have different sagittal values; which are the true measures of curves
in lenses. Add in aspheric curves (every manufacturer has different formulas
for the curves) and the old rule of 'match base curve' has no real meaning
when used with modern lenses. It's just something that is left in the
curriculum for the course OPT 208. (One school's listing for the generic
course of basic ophthalmic optics.)
THe OD that said there is no peripheral distortion in the distance should
try backing a car up through a set of poles while wearing them. That OD will
be in for an awakening. There is a definite amount of distortion present in
all brands. I check them out regularly whenever a rep tells me that they
eliminated said distortion and it's yet to happen!
Carl
<hol...@cneuro.zool.ohiou.edu> wrote in message
news:FM2LE...@boss.cs.ohiou.edu...
SpecOPTICS
>and magnification of a lens.
Tsk Tsk....:-).. if this is the "reason" your lab is giving you on "why" they
use base curves they are feeding you BAD information :-).. The reason base
curves are selected are covered in a number of text books, one of the better I
have seen is Robert Tylliers text book (which an OD borrowed from me and NEVER
returned LOL)
>Matching base curves is generally a good
>idea
Another Tsk Tsk.. :-) Usually if you are doing that its for cosmetic reasons
NOT optically quality reason. I have a number of accounts where the OD's are
always rquesting the lens be ground onto a far "flatter" base curve then needed
to help with the "profile" of the lens. This creates MAJOR oblique astigmatism
in a lens when you go off a needed curve and narrows the working visual field
of the lens.
The biggest problem here is a uneducated optician giving someone something
just because , either they wanted it or because you can make more on a PAL then
a flat top bifocal or tri' !! :-)
All that information we get from the manufacture's are not given to us lab
guys for "box" filling when we ship lens..LOL The main problem is that a PAL is
a semi-aspherical design so we are already fighting a major oblique problem in
power and axis then on top of that throwing in a RX of -10 something?.. no
wonder his world looked like a fishbowl... If there ever was a person who
needed a high index bi or tri this is the guy! you can even get bi-focals in a
-2&-4 concave design in an index of 1.60..I would have went that route a -2
front curve with a good anti reflective coating to help with the surface
reflections. You wanted a flatter ocular curve and have access to a wider
visual field and the bi-concave is not bad enough to cause any type of tunnel
vision.
Jeff
(not an OD but atleast an ABOM,NCLE and wholesale lab owner :-)
hol...@cneuro.zool.ohiou.edu
answers, my old Physics book, and searching the sola and varilux
web sites, I think I understand the issues involved much better.
I'll followup here.
If I understand what Jeff and Carl have said, a simplified formula
is: sphere minus base curve should equal -6 for minimum peripheral
distortion.
So, for example a sphere of -10.5D and base of .5 gives
-10.5 - 0.5 = -11 which is far from -6. If the base were 3, then
-10.5 - 3 = -13.5 which is farther from -6 and so this would seem
to give more distortion in the periphery (theoretically). So although
it is virtually impossible to have no distortion with a -10.5D Rx in
a progressive lens, at least the .5 base curve "should" be better than
the steeper 3.0 base curve. Is this basically correct?
I found the sola web site ( http://www.sola.com ) yesterday. In one
part they have a plot of base curve vs. lens power which suggests that
a power of -10 should have a base curve of about 2. Elsewhere they
have a progressive lens centration chart for their lenses and they
recommend a base curve of .5 for a -10.5D prescription for the actual
lenses I have now (it turns out that my latest lenses are from sola
and they are in this chart). This at least suggests some thought went
into choosing the .5 base curve I have now rather than just "matching
my old base curve", or else "they just used whatever base curve
they had in stock" which is the answer I got when I questioned the
3.0 base curve.
I also gather from Kevin's comment that progressives take much longer
to adapt to than single vision glasses. I take it that a month is
probably not unusual with a -10.5 Rx. When the OD's office calls
back, I'll tell them I'll wait a bit longer to see if I can adapt.
Finally, Carl, is that course "Geometric optics in opthalamic design"
on the web somewhere? I can handle the math and I might like to
look that over sometime.
--Bill
DMeisters
>clears up the confusion for the people able to handle the math.
>
>A -10.50 on a 0.50 base curve would need about a -11.00 back curve. Well,
>this is far from the -6.00 back curve ideal. The farther you get from -6.00
>back curve, the worse your peripheral vision is the general rule.
If I understand you correctly, you're advocating that the lens would perform
better with a -6.00 D back curve as opposed to a -11.00 D back curve. However,
John Davis (the author of the article you referenced) would probably say that a
0.50 base would be nearly perfect for that power. The oblique astigmatism
introduced at 30 deg off axis would be almost zero with that back curve.
Producing that Rx with a -6.00 back curve, though, would introduce over 2.50 D
of oblique astigmatism at 30 deg off-axis.
I think the old rule of thumb was Base = 1/2 X Sphere + 6.00. For a -10.50 D
sphere power, this would be 1/2 X -10.50 + 6.00 = +0.75 Base.
Anyway, if I misunderstood you please disregard my response.
Best regards,
Darryl
Specs31
>introduced at 30 deg off axis would be almost zero with that back curve.
>Producing that Rx with a -6.00 back curve, though, would introduce over 2.50
>D
Darryl
I don't know the PAL's you have been using but did you ever see what that
type of power does distortion wise? I know you guys tell us "lowly" lab owners
:-) how much power these lens "can" take but it is hard explaining to a patient
why they feel like they are in the bottom of a fish bowl :-)
You already have a higher degree of periphrel distortion in a PAL then
compound that problem by not using a proper base curve...YIKES...
BTW I think you didn't get the posting..Carl wasn't saying THIS rx with a
back curve of 6^ ...that would be crazy, the only way you would be able to get
that is have a bi-concave cut..talk about a nightmare.. a PAL in a (-)4 bi
concave!!LOL The optimum curve we shoot for is a -6 (ocular).. or as close as
possible, of course that isn't accomplished all the time but it is the target
:-) I know coulda answered but what the heck.. it's a Sat. and not many
football games on :-)
Jeff
William Stacy
BillyFish wrote:
>
> Not being knowledgeable in ophthalmologic design, what is a base curve?
>
> Bill
Ophthalmic lens blanks usually come from the manufacturer with the front
surface "finished" (ground and polished to one of many curvatures
available) and the inside surface just roughly cast. The particular
curvature of the finished surface (expressed in diopters), is the lenses
"base curve". There are charts and formulas that tell the lens grinder
what base curve is best for what final Rx, and what curvature to grind
the inside surface to that will result in the specified Rx.
Bill
BillyFish
Brad Rogers
>>.50 base would be nearly perfect for that power. The oblique astigmatism
>>introduced at 30 deg off axis would be almost zero with that back curve.
>>Producing that Rx with a -6.00 back curve, though, would introduce over 2.50
>>D
>
> Darryl
>
> I don't know the PAL's you have been using but did you ever see what that
>type of power does distortion wise? (snip)
Jeff,
I seem to remember having this argument with you about a year ago.
Changing the base curve to reduce oblique astigmatism usually
_increases_ distortion - not reduces it.
Anyway, I do understand the general spirit of what your saying.
Regards
Brad
DMeisters
I don't know... I would contend that an 0.50 base (or at least something close
to that) is the proper base curve for a -10.50 D sphere. (This is, of course,
contingent upon whether or not the lens blank is thick enough and the
manufacturer recommends using an Rx that high.) That base curve will have the
least amount of peripheral aberrations. I think you will find that all of your
progressive/single vision/flat-top manufacturers would recommend using a very
flat base (e.g., 1.00, 0.50, etc.) for that power. And, as you've noted
yourself, although it is possible to shoot for a -6.00 D back curve in plus
powers it is very impractical and optically unsound to do so in minus powers.
The barrel distortion produced by that high of a minus power, although
initially annoying for some myopes after an Rx change, is generally adapted to.
Moreover, they would have to deal with this distortion in virtually any lens
design. Even if progressives may exacerbate that effect, I recall very few --
if any -- high myopes rejecting progressive lenses back in my dispensing days.
Best regards,
Darryl
DMeisters
>_increases_ distortion - not reduces it.
I believe that, in general, steepening the base curve will reduce distortion --
even if oblique astigmatism is increased or decreased.
Best regards,
Darryl
BillyFish
>Date: Sun, 05 December 1999 12:30 AM EST
>Message-id: <3849F858...@obase.net>
>
>
>
>BillyFish wrote:
>>
>> Not being knowledgeable in ophthalmologic design, what is a base curve?
>>
>> Bill
>
>surface "finished" (ground and polished to one of many curvatures
>available) and the inside surface just roughly cast. The particular
>curvature of the finished surface (expressed in diopters), is the lenses
>"base curve". There are charts and formulas that tell the lens grinder
>what base curve is best for what final Rx, and what curvature to grind
>the inside surface to that will result in the specified Rx.
This sounds a lot like "bending" a lens. That is, adjusting front and back
surface radii to minimize aberration. If I am gett the hang of it, the
prescription tells what the difference should be between the custom inner
surface and the base curve?
Brad Rogers
Agreed . . .
and "traditional" point focal lens design will eliminate oblique
astigmatism with curves which are usually quite steep.
Incidentally, there is a unique case where oblique astigmatism and
distortion are both at a minimum at the same time. This is where the
Rx is at about -20.00 D (made with a plano base).
Regards
Brad
Specs31
>prescription tells what the difference should be between the custom inner
>surface and the base curve?
>
Now you're cooking!!! :-)..BUT lets make it even more COMPLEX :-)..besides the
base curve effecting the amount of ocular curve, the actual lens material can
also do it as well :-) ..So if I have a 6^ base lens in CR39, 1.55 mid index
and a 1.60 high index even though they are all being cut on the same base
curve to get the same power in grinding it would have THREE different ocular
curves curves!! LOL
Sometimes right when you think you are getting it we throw in another set of
rules for you to ponder :-)
But basically you have the basics...the base curve is what you would get in
power if you cut a flat surface on the ocular side (back)....
We are talking base curves but on the "lab side" we talk true curves...so
even though we say "6 base" the "6" in that phrase is not exactly
true..depending on the maker you could have a 5.92 (Sola) but labeled 6 base.
Silor's 6.25 is actually 6.20 and Younger is a 6.30 but labeled
6.50..LOL..how's all this for "confusing the mix" ? :-)....it's just common
practice among the 3 O's to round off the numbers when conversing.. makes it a
little easier to pass information between ourselves...the actual "true" curve
is only important to us lab guys that are doing the grinding..
Keep this up though and we'll let you set for the ABO..that is unless you
want to go to med. school..:-)..or (God forbid) become a lab rat like me..
Jeff
DMeisters
>distortion are both at a minimum at the same time. This is where the
>Rx is at about -20.00 D (made with a plano base).
Yes, a graph of the positive and negative roots to the solution for point focal
forms (called a "Tscherning's ellipse") converge at around -22.00 D in the
minus end. The positive solutions, or Wollaston curves, are much steeper than
the negative solutions, or Ostwalt curves -- which currently serve as the basis
for modern corrected curve lenses. Although any power within this range can be
made on either the Ostwalt or Wollaston curves to completely eliminate oblique
astigmatism, the steeper Wollaston curves produce less distortion. Around
-22.00 D is also the only power completely free from both oblique astigmatism
and mean power error simultaneously. (All other powers will suffer from some
residual mean power error.)
Best regards,
Darryl
BillyFish
>Date: Mon, 06 December 1999 07:35 PM EST
>Message-id: <19991206193543...@ng-fb1.aol.com>
Although I am mostly familiar with lenses such as camera lenses where the image
is created at the focal plane, I am reasonably convinced that the following
applies in other situations. Aberration is minimized when the refractive
change in angle at any surface is about the same as at any other angle. This
would be influenced by the index of the material and the conjugated points bein
imaged to one another. For glass with index in the neighborhood of 1.5 being
used as a camera lens, it turns out that a plano-convex lens is very close to
optimum. It often is not worth the economic effort to modify the plano
curvature to obtain absolute minimal spherical aberration when an off-the-shelf
plano lens would do the job.
Bill
William Stacy
BillyFish wrote: For glass with index in the neighborhood of 1.5 being
> used as a camera lens, it turns out that a plano-convex lens is very close to
> optimum. It often is not worth the economic effort to modify the plano
> curvature to obtain absolute minimal spherical aberration when an off-the-shelf
> plano lens would do the job.
One problem with a plano curve is that it is flat, like a mirror, and reflections
(direct and internal) may be more noticeable/annoying.
Bill
BillyFish
>Date: Tue, 07 December 1999 12:48 PM EST
>Message-id: <384D4852...@obase.net>
That annoyance may be true for ophtalmalogic lenses but is often not true for
other applications, especially if the lenses are antireflect coated. The main
reason for using plano-convex lenses, which usually are not quite optimum, is
that they are relatively cheap and readily available.
Opthalmolagic applications do not usually require the figure required in other
applications where high resolution, near diffraction limit, and large aperture
are required simultaneously. Getting an optical house to grind and polish a
few custom lenses for which they have no test plates is usually an expensive
proposition that makes even the most expensive spectacle boutique look cheap by
comparison.
Bill
Specs31
>a progressive lens, at least the .5 base curve "should" be better than
>the steeper 3.0 base curve. Is this basically correct?
yes, (how's that for short and sweet?_ :-)
You didn't mention the type of material that was used, it could be CR39 or a
high index material...the higher the index of refraction of the material the
less actual ocular (backside) curve you need to still get the same RX power in
the lens.
>have now rather than just "matching
>my old base curve", or else "they just used whatever base curve
>they had in stock" which is the answer I got when I questioned the
>3.0 base curve.
Sounds like the reason I hear most often..but being a wholesale lab I have a
wideer degree of stock then chain stores keep..my stock of semi-finished and
finished lens run around $45,000..I hate having to keep that much money tied up
sitting on a shelf but people tend to hate to wait that one day it would take
for me to order it :-)
>Finally, Carl, is that course "Geometric optics in opthalamic design"
>on the web somewhere?
I doubt that Amazon maybe able to track down a copy for you when checking
another good text is by Robert Tillyer on "base curve selection"..it's been out
of print for years but it's good as gold for us lab guys :-) I loaned my copy
to an OD and I still havn't got it back :-)
Jeff
Brad Rogers
> It often is not worth the economic effort to modify the plano
>curvature to obtain absolute minimal spherical aberration when an off-the-shelf
>plano lens would do the job.
. . . Except that spherical aberration is NOT an aberration that
affects the wearer of spectacle lenses - since the pupil acts as a
natural aperture and thus only makes use of a very small part of the
lens at any one time.
Other aberrations produced by spectacle lenses, like chromatic
aberration, rotatory and ordinary distortion and oblique astigmatism
are all affected by lens design and since the camera lens focuses onto
a flat plane, (and the retina is curved) it doesn't neccesarily follow
that a plano based ("flat") lens will remove all the aberrations. If
anything flat lenses on the whole perform pretty badly( unless they
are aspheric).
Other aberrations like (Petzval) curvature and Coma are (like
spherical aberration) minimised considerably because of the pupil.
Brad