# Optical Forums > Ophthalmic Optics >  abbe values and chromatic aberration

## mirage2k2

Just a few questions that might turn into an interesting thread ...

What exactly is it about a material that determines its abbe value? I know it is proportional to the index of the material but there is more to it. Is it to do with the resonant frequency of the material ... and the speed at which it refracts the different colors?

What is the abbe value of 1.8 glass? I've seen many different values ranging from 25 to 38. The reason I ask is because I consistently see more color seperation in 1.74 plastic lenses (abbe 33) than I do in the 1.8 glass ... I've compared a lot of them ... all very similar rx and the plastic is always worse - sometimes a lot worse!  

Finally, my experience has been that the amount of color seperation in the 1.74 plastic is quite inconsistent ... having many pairs (and spares) I notice that when comparing a number of lenses, all with the same/similar rx, they have differing amounts of seperation. Can someone explain this to me?  Is it because of processing aberrations? Is it to do with the quality of the blanks, i.e. great batch versus average (friday afternoon) batch?

Just guessing, when the manufacturer states a material has an abbe value of 33 ... perhaps they mean 33 +-15% ... sound reasonable?

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## lensgrinder

> What exactly is it about a material that determines its abbe value? I know it is proportional to the index of the material but there is more to it. Is it to do with the resonant frequency of the material ... and the speed at which it refracts the different colors?


 
The Abbe number or constringence is the reciprocal of the dispersive power of the material or how much white light is broken up into colors. Abbe number is used to find out how much transverse chromatic abberation there would be at a certain point on a lens. The threshold value for TCA is 0.1, anything less is not considered a problem.
TCA=p/v
where:
TCA is Transverse Chromaic Aberation
p is prism
v is v-value or Abbe Number
So let's say you are dealing with a plastic lens with v-value of 58 you would have to look through 5.8D of prism to before you see color fringes. On a 3.50D lens, this is 16mm away from center. You would only have to look through 3.0D of prism on a lens with a v-value of 30.




> What is the abbe value of 1.8 glass? I've seen many different values ranging from 25 to 38.


 It would depend on whether it is 1.8,1.83,1.88, but from what I have seen it should be between 30-35.

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## mirage2k2

Thanks a lot for helping lensgrinder:cheers: 




> So let's say you are dealing with a plastic lens with v-value of 58 you would have to look through 5.8D of prism to before you see color fringes. On a 3.50D lens, this is 16mm away from center. You would only have to look through 3.0D of prism on a lens with a v-value of 30.


I should look at Darryls article on chromatic aberration again to figure out the math.  The lenses I'm comparing in this thread are around 6D and have between 2-4D base out prism ... so as you can imagine there is quite a lot of seperation and because of the prism you don't have to look away from the oc at all to see it.




> It would depend on whether it is 1.8,1.83,1.88 ...


Pretty convinced it is 1.8.

What I'd like to know is why the plastic is worse than the glass ... my findings are ... a 1.8 glass with 3 base out prism displays similar color seperation to a 1.74 with 1.5 prism ... and is MUCH better than a 1.74 with 3 prism.

I'd also like to know why the 1.74 plastics are so inconsistent - 2 identical lenses displaying quite different amounts of seperation.  I'm drawing my own conclusion that 1.74 is a bit of a handful to work with ... and possibly inappropriate material for +6 3 base out ... but what do I know!

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## Darryl Meister

> What exactly is it about a material that determines its abbe value? I know it is proportional to the index of the material but there is more to it. Is it to do with the resonant frequency of the material ... and the speed at which it refracts the different colors?


Yes, as I note in the article, the resonant frequency of transparent materials is in the violet end of the spectrum. This means that colors in the blue end of the spectrum will do more "work" on the atoms of the material as light is absorbed and then re-emitted by these atoms, which in turn causes these waves of light to slow down more. The slower the light travels, the higher the refractive index becomes. Further, as the refractive index increases, light is deviated or "refracted" more by the material as it passes through. Various ingredients can be added to lens materials to increase their refractive index, such as metallic oxides in glass or sulfur and aromatic structures in plastic, but this generally increases chromatic aberration.

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## mirage2k2

Thanks Darryl. :cheers: 

I'm trying to figure out why some lower index materials, like poly, have lower abbe values than some higher index materials, like hoyas 1.7. In theory the poly should have a higher abbe since it has a lower index. There must be some unique property of poly that gives it such a low abbe, and some unique property that gives the 1.7 such a high abbe.

What kind of aberrations are processing aberrations? Can problems in processing generate more chromatic aberration in a lens?

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## Darryl Meister

> In theory the poly should have a higher abbe since it has a lower index. There must be some unique property of poly that gives it such a low abbe, and some unique property that gives the 1.7 such a high abbe.


While it is true that higher index lens materials will generally have lower Abbe values, there is no one-to-one correspondence between refractive index and Abbe value. Ultimately, it just depends upon the chemical makeup of the material. Also, glass materials generally have higher Abbe values for a given index than plastics (but remember that the index is increased by completely different means).




> What kind of aberrations are processing aberrations? Can problems in processing generate more chromatic aberration in a lens?


Processing aberrations include "waves" (or local irregularities in the shape of the surface), as well as various other "defects" related to the smoothness and/or luster of the surface. They do not affect chromatic aberration.

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## mirage2k2

Thanks again.

Since I have your ear, any comments on why I'm seeing such inconsistencies in chromatic aberration in the 1.74?

In a previous thread you mentioned that some of the MR materials used in 1.67 were very robust ... is 1.74 in this category or is it a little less robust?

Did you see the post I made about *Rx Compensator v2.3* in the file directory?

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## Darryl Meister

> Since I have your ear, any comments on why I'm seeing such inconsistencies in chromatic aberration in the 1.74?


You'd have to elaborate. Assuming that they are the _same_ material (which means same Abbe value), there shouldn't be a difference in chromatic aberration if all other factors (frame, power, etc.) are also equal. And I'm only aware of one 1.74 high-index material, which has an Abbe of 33.

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## mirage2k2

> You'd have to elaborate. Assuming that they are the _same_ material (which means same Abbe value), there shouldn't be a difference in chromatic aberration if all other factors (frame, power, etc.) are also equal.


They are the same material and rx with exception to amount of prism ... what is confusing me is this ... I have a 1.74 with 4 base out prism and a 1.74 with 3 prism.  In theory the one with 4 should have more chromatic aberration ... but not so ... it is a noticeably better!  In fact, this lens with 4 prism is better than most of the others I have!  I have a feeling it was surfaced in Japan ... so stringent quality control comes to mind.  This is why I asked about processing aberrations.  I just think this lens has been machined to a higher standard than all the others that have been done here in Oz.




> And I'm only aware of one 1.74 high-index material, which has an Abbe of 33.


Which one?  These are all Nikon Lite.

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## Robert Martellaro

> What is the abbe value of 1.8 glass?


Zeiss Lantal 1.8 (Lanthanum flint glass) is 35.4. 




> They are the same material and rx with exception to amount of prism ... what is confusing me is this ... I have a 1.74 with 4 base out prism and a 1.74 with 3 prism.


Perhaps one of these lens pairs is not 1.74. A 1.70 index lens might have been used without you or your optician knowing it, and that might be a good thing considering that 1.70 has a higher abbe number and is equal to if not lighter in weight than 1.74 (if the asphericity is the same). Differences in lens position could also account for varying degrees of blur/color.

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## Darryl Meister

> They are the same material and rx with exception to amount of prism ... what is confusing me is this ... I have a 1.74 with 4 base out prism and a 1.74 with 3 prism. In theory the one with 4 should have more chromatic aberration ... but not so ...


The lens with more prism will produce more chromatic aberration, if all other things are equal (including the frame and how it fits, the Base curve of the lens, and so on). If you have a plus prescription, I could see how you might perceive less chromatic aberration, since the prism would be somewhat neutralized as you use the periphery of the lens. However, if you have a minus lens with base out prism and still experience more chromatic aberration, you should ask your local optician to have a look at them.




> This is why I asked about processing aberrations. I just think this lens has been machined to a higher standard than all the others that have been done here in Oz.


Processing aberrations may affect your quality of vision, but they will not affect chromatic aberration -- unless there are regions of significant blur caused by extreme power variations, which is unlikely and would probably be more bothersome to you than the chromatic aberration.

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## mirage2k2

> If you have a plus prescription, I could see how you might perceive less chromatic aberration, since the prism would be somewhat neutralized as you use the periphery of the lens...


Isn't this only true between oc and temple? I thought it was the opposite between the oc and the bridge (the problem area) ... since this part of the lens is just another base out prism.




> Processing aberrations may affect your quality of vision, but they will not affect chromatic aberration


My thinking is that chromatic aberration is down to the chemical/molecular make up of the material ... if this material is subjected to excessive heat during processing then I was guessing that this might mess around with the material on a molecular level and perhaps generate more chromatic aberration.

Anyway, thanks for all of your help ... I should probably stop pestering you with all these questions. I'm going to the bookstore today to pick up a copy of "Geometrical, Physical, and Visual Optics" by Keating ... this should keep me out of trouble for a while :)

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## Darryl Meister

> Isn't this only true between oc and temple? I thought it was the opposite between the oc and the bridge (the problem area) ... since this part of the lens is just another base out prism


It is true in both cases. However, since lenses are generally decentered in a few millimeters, the eye can traverse a greater distance between the OC and temple, before running out of lens. This is where having a local optician examine your lenses and isolate your complaint would be beneficial.




> My thinking is that chromatic aberration is down to the chemical/molecular make up of the material ... if this material is subjected to excessive heat during processing then I was guessing that this might mess around with the material on a molecular level and perhaps generate more chromatic aberration.


It is very unlikely. Besides, even if heat were somehow to affect dispersion by a significant amount, it would most likely also affect the nominal refractive index of the material, which result in off-powers.




> Which one? These are all Nikon Lite.


I just noticed that you aren't in the United States. There is a second 1.74 high-index material available _outside_ the US, though the Abbe values of these two materials are probably similar. I'd have to look into it. However, I believe Essilor's Nikon product uses the same 1.74 material that is available here in the US.

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## mirage2k2

> This is where having a local optician examine your lenses and isolate your complaint would be beneficial.


They have been examined on a number of occasions - but only to determine correct rx, prism and centres, etc. ... perhaps I should specificaly ask them to look for chromatic and processing aberrations ... and put my mind at rest :)

From a consumer standpoint I've already decided that 1.74 is great for my casual wear specs, but not good enough for my work specs.

Anyway, for me this thread has been very beneficial from a learning standpoint and I'm sure others will glean lots from it also.  Thanks for all of the input.

:cheers:

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## mirage2k2

Darryl, you've confused me ... you've said that in a plus lense with prescribed base out prism, the prism is neutralized when looking off-axis between the OC and the bridge. Is this what you meant  :Confused:  

I can understand the prism being neutralized between the OC and the temple because when looking through this part of the lens there is induced base in prism ... and this will neutralize the prescribed base out prism. But when looking through the OC-bridge part of the lens there is induced base out prism ... surely this will add to the prescribed base out prism ... not neutralize it  :Confused:

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## Darryl Meister

I don't recall seeing your prescription in this thread.

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## mirage2k2

> I don't recall seeing your prescription in this thread.


Your right - its not :) 

My rx isn't important - I'm just trying to get my head around some of the stuff we discussed in this thread :o

So, in a plus lens with base out prism, is it correct to say that the prism is neutralized when looking between OC-temple (because of induced base in prism) and quite the opposite when looking between OC-bridge - where there is both prescribed and induced base out prism  :Confused:

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## Darryl Meister

> My rx isn't important - I'm just trying to get my head around some of the stuff we discussed in this thread


But since we're discussing the problem with _your_ lenses in this thread, your Rx is in fact very important.




> So, in a plus lens with base out prism, is it correct to say that the prism is neutralized when looking between OC-temple (because of induced base in prism) and quite the opposite when looking between OC-bridge - where there is both prescribed and induced base out prism


This requires an understanding of prism in spectacle lenses. Base out prism in a Plus lens is produced by moving the optical centers out from the lines of sight. Base out prism in a minus lens is produced by moving the optical centers in.

For most frames, the optical centers of the lenses are decentered in 1 to 4 mm in order to align them with the lines of sight (your "PD"). This means that you have a greater distance between the optical center of each lens and the outer (temporal) edge of the frame. However, with Base out prism in a plus lens, this distance is reduced (of course, enough Base out prism will make the distance to the nasal edge of the frame considerably larger).

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## mirage2k2

I think I've come to a gun fight with no gun :D ... as one of the other optiboarders so nicely put it.

I've just picked up a pretty scary looking book on optics ... I'll spend some time in it and then perhaps I'll have a small pistol :D 

My limited understanding of lenses is this ... a plus lense is equivelant to 2 prisms base to base and a minus ... apex to apex - I stick to plus lenses in threads ... I find them easier to understand because I'm wearing them.
Leaving prescribed prism aside, induced prism in a plus lens is base in when looking through OC-temple (temporal) and base out when looking through OC-bridge ... please reassure me that I at least have this much correct  :cry:  


I do have an intrigueing question about prescribed prism ... we'll use a fairly extreme amount, say 6D base in (or out) in one lens. Now I look straight ahead and what I see is an image that has been shifted by 4-5mm (I don't know the exact amount) ... so the bent light has traversed the lens at an oblique angle ... so my question is ... will this generate any degree of oblique astigmatism since it is very similar to looking off-axis at an oblique angle?

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