Abbe Value

[Edgley Gonzaga]

Hello Folks

I would like to know how do the eye perceive the cromatic aberration? Is it true that the Abbe value goes till 100?

If i am not wrong the eye's abbe is about 43, so a lens which has an upper abbe doesnt bother the vision perceived by the eye, is it?

thanks very much

Edgley

[David Wilson]

Hi Edgley
The eye has relatively high axial chromatic aberration (between 1 and 2 dioptres, depending on the wavelength of red and violet that you use). However, spectacle wearers are not troubled by axial chromatic aberration. because the ye's own chromatic aberration is greater, they are affected by transverse chromatic aberration (TCA). This is the chromatic aberration created by the the prismatic effect at the periphery of a lens for excentric gaze. TCA is not noticed (according to Jalie) below 0.1 primn dioptre of TCA. The formula for TCA is cF/V. Where c is the distance from the OC in centimetres , F is lens power and V is the Abbe number. According to Torgersen, acuity is not affected until TCA reaches 0.16 prism dioptres and then it drops by one line on the Snellen chart.

Abbe number and TCA is overstated as a problem for spec wearers. It is only a consideration for powers of more than +/-4D and then only at the edge of the lens. As a +6.75D hyperope (with a +1.5D add) wearing lenses of Abbe 36, I can say that, if well fitted, Abbe should not be a problem for most spectacle wearers, especially those below +/-4D. Torgersen and Jalie provide formal evidence of my anecdotal experience.

I hope this is of some use, Edgely.
Regards
David

[Susan Henault]

"How does the eye perceive chromatic aberration?"

Basically, images that are viewed well off of the optical center (area of prism) become fringed with color. There will be a reddish hue to one side and a blue/violet hue to the other. The result is less clear vision. The wearer may not be able to specifically identify the "color", but images off center will appear smeared, or smudged (less clear/sharp) due to this affect.

Chromatic aberration is very real and some patients are more sensitive to it than others. While many cite that patients falling within the +/- 4.00D range should not experience problems, there are some exceptions, particularly among hyperopes.

Many believe that adding an A/R coating to a low abbe material will reduce or eliminate the affect of chromatic aberration. This is NOT true. A/R does eliminate reflections (which tend to be worse in high index than in standard plastic or glass). But A/R does nothing to change the chromatic properties of the material. The goal is to minimize any factor that may contribute to adaptation issues. A/R does eliminate a big one -- increased reflections.

[chip anderson]

Susan: All the patients I have seen with problems of chromatic abberation saw yellow and red outlines. None of the mentioned blue. Usually this was due to our failing to notice the proper vertical center heights. This would of course show up sooner and more pronounced in materials with poor ABBE values.


Chip

[yzf-r1]

Chip,

I have a theory as to why your patients did not report any blue fringes.

Both blue and red are present at the same time (i think it is blue at a dark-light interface, and red at a dark-light interface), but the red is much more distinct and noticeable, they ignore the fact that there is also blue finges present. Also the eye is less sensitive to blue anyway. The reason why it is either yellow or red is because, the further away from the OC, the worse the aberrations get, and when you are fairly close, the red is a very faint band, and all that is seen is appreciated is yellow. And other times, when they are looking futher off-centre they see the red. In other words, they are totally ignoring 50% of the fringes, and only concentrating on the red end.

yahya

[yzf-r1]

...also the blue is on the dark side of the dark-light border whereas the yellow-red is on the light side so the latter is easier to see.

yahya

[Optical Plumber]

David Wilson said:
The eye has relatively high axial chromatic aberration (between 1 and 2 dioptres, depending on the wavelength of red and violet that you use). However, spectacle wearers are not troubled by axial chromatic aberration.

What is axial chromatic abberation?

David Wilson also said:
... Abbe number and TCA is overstated as a problem for spec wearers. It is only a consideration for powers of more than +/-4D and then only at the edge of the lens...

How does Rx prism or progressive thinning prism affect TCA? I recently got my 3rd pair of progressives and I immediately noticed some TCA. I had never experienced this before. My rx changed from -.75 sphere +1.25 add to -.75 with a +1.75 add. In addition there is +1.50 diopters base down prism (for cosmetic thinning) in the current Rx. I don't remember if any of my previous specs had prism thinning. Would that by itself be enough to create an obvious TCA effect? After a couple of months, I stopped noticing the TCA and have adjusted well to the new glasses, but I can still see it if I look at a dark edge such as a tree line or a roof line against a light sky. Would this much TCA (or prism) reduce the acuity of my lenses significantly?

Terry

[Jeff Trail]

Optical Plumber said:
What is axial chromatic abberation?


What is axial chromatic abberation?

You have two types of chromatic aberration, longitudinal and lateral, longitudal (axial) chromatic aberration each wavelength has a slightly diffeent amount of refraction at the same surface curve axial chromatic aberration results in a series of points spread out along the optical axis, axial aberration is not directly connected to prismatic effect (power) so a plano would have no longitudinal (axial)chromatic aberration.
The main trick is that the index of refraction we use for a material is only based upon one wavelength (yellow in the U.S.) and each length would change the index slightly as you went through the spectrum, the larger the diference as you go through the spectrum the lower the abbe value the higher the chromatic effect.. the more increased angle and magnification will also have a direct effect as well, say you have a +4 lens (using David's example) you could actually vary the amount by using panto and vertex..increased field of physical lens surface in relation to the eye the greater the effect of chromatisism...
This is one of the times me and David part paths, he mentioned some powers, where, I think, angle and power and vertex also comes into play, and you can experience those problems a lot sooner than he has stated ..especially in hard line segs and "letter bi-fringence" one of the main complaints I hear most often..oh they might not call it that but "letters look a little fuzzed and sometimes outlined by a color" ..
I think as far as straight on optic theory David's would work but when you include the other factors of critical angle and segs and power distortions vertex and light sources and any number of other things that come into play on a daily basis that +4/-4 rule might be a bit stiff ;)
Not saying anyone is wrong, just some observations I made and seen..

Jeff "grind'em if ya got'em" Trail

[chip anderson]

If we really care about this sort of thing why hasn't someone come up with a composite lens with more than one index. The strong lenses in telescopes and microscopes, the better magnifiers (and some optical instruments) get away from this with lanimated lenses useing crown and flint.

Lots of the fused glass bifocals used to take this into concidersation.

Chip

[Jeff Trail]

Technically ALL lens have more than one index, that is "why" you have axial chromatic aberration, you can reduce it by using spectrum manipulation but than again who wants a "colored" lens like that? would not be to popular..we just happen to pick a wave length to base our "index" on..in the U.S. it is yellow..
Optics like photography and telescopes and such do have some advantages, they do not have to worry about cylinder and the do not have to worry about the vertex relation and physical lens surface like we do, you can mix and match lens designs in multiple stacks saddle back, convex, concave etc., etc...why we have to worry about vertex and axis of rotation and full visual field..while in all those you worry about a line of sight and not critical deviation, laws of refraction, laws of reflection like we do in ophthalmic's..well they would (reflection and refraction) but since they are usually all working in an enclosed area they get to go about different way than we do..
Maybe an optical engineer will like to take a stab at it and explain further..I could go through my texts and give you a ton of formula's on top of what I just posted, but I'm not where they are, and still to full of turkey and lazy to go into a ton of research..:-)

Anyone else feel as "lazy" as I do? :-)

I'll post the technical fill in stuff if you really just have to have it..but not during any football games :-)

Jeff "DON'T feel like grind'em even if I had'em" Trail

[Darryl Meister]

Well, I happen to have some spare time on my hands, so I'll tackle this topic on chromatic aberration and the eye, as well. Much of this was posted in one form or another by several other knowledgeable OptiBoarders in this thread, so I apologize if some of this is redundant.

First of all, chromatic aberration occurs because different colors (or frequencies) of light travel through materials at different velocities. Essentially, the blue end of the color spectrum is closer to the "resonant frequency" of the molecules of common lens materials, so blue-ish colors do more "work" -- and therefore travel more slowly -- than red-ish colors. Since the refractive index of a lens material is the ratio of the speed of light in air to the speed of light in the material, and each color travels at a slightly different speed, this means that each color will have its own unique refractive index for a given material. Blue-ish colors will have a higher refractive index than red-ish colors.

This means that these colors will also be refracted and focused differently by the lens material, as well. For instance, since blue light has a higher refractive index than red light, blue light will have a shorter focal distance and will be refracted more in the presence of prism. This results in dispersion, or the breaking up of white light into its component colors after refraction by the lens. Chromatic aberration is simply a measure of this color dispersion. The greater the difference between the refractive index of the blue end of the color spectrum and the refractive index of the red end for a given material, the more the material will disperse colors, and vice versa.

The degree to which a given lens material will disperse light is described by a measure of its refractive efficiency or, more commonly, its Abbe value (after Ernst Abbe). Lenses with a high Abbe value will disperse light less, and produce less chromatic aberration, than lenses with low Abbe values. In general, high-index materials often produce considerably lower Abbe values than either hard resin (at 1.500) or crown glass (at 1.523), which makes these materials more likely to produce symptoms of chromatic aberration. Fortunately, some newer high-index materials have been engineered with higher Abbe values, minimizing this problem.

There are two measures of chromatic aberration (or dispersion): axial (related to focal power) and lateral (related to prism). Axial chromatic aberration, which is a measure of the difference in focus between the blue and red ends of the color spectrum, is calculated by:

AC = F / v

where (F) is the power of the lens and (v) is the Abbe value. Similarly, lateral chromatic aberration, which is a measure of the difference in prismatic deviation between the blue and red ends of the color spectrum, is calculated by

LC = P / v

where (P) is the prism of the lens (given by decentration times power). Consequently, both the axial and lateral chromatic aberration of a lens increase as the Abbe value decreases. Since the amount of color fringing caused by lateral chromatic aberration will increase as the prism increases, this illustrates the importance of choosing well-fitting frames that keep the lenses nicely centered in front of the eyes. (The farther the eye is positioned from the optical center, the more prism and lateral chromatic aberration produced.) This also illustrates why excessive prism-thinning in a high-index progressive lens might produce slightly more color fringing at some angles.

Axial chromatic aberration results in some colors being "out of focus," while lateral chromatic aberration results in the familiar "color fringing" around objects (such as the blue and amber fringing around a white fluorescent bulb). The human eye suffers from both forms of chromatic aberration, and actually has about 1.00 D of axial chromatic aberration. This is actually what allows the common "duochrome test" to refine a spectacle refraction. You can demonstrate the lateral chromatic aberration of your own eye by slowly sliding a card up over your pupil while staring a bright white object.

Since each color has its own focal length and focal power, a "reference color" -- or reference wavelength -- must be chosen in order to label a lens with a "single power." This is also why it is particularly important to set your automatic lensmeter correctly when measuring high-index and polycarbonate lenses. In the United States, this reference wavelength is a yellow-green color produced by electrically excited helium gas. This means that when we quote the power of a lens in the US (e.g., "+4.00 D"), we are actually quoting the power of a yellow-green color. In Germany and Japan, it is a greenish color produced by electrically excited mercury gas. These colors lie close to the center of the color spectrum and to the peak of the color sensitivity of the eye, which is why they were chosen.

There are a couple of techniques you can use to minimize chromatic aberration. Achromatic doublets, or composite lenses combining a low-index and a high-index lens, are very common in precision optics. However, these lenses are too complex, too heavy, and too expensive to be used for spectacle lenses. For spectacle lenses, the most common ways to minimize chromatic aberration include using materials with a high Abbe value, choosing frames that keep the eyes well centered (to avoid prism), adjusting the frame properly, and selecting the correct base curves (which minimizes the additional blur produced by other off-center aberrations).

Best regards,
Darryl

[Jeff Trail]

yea, what Darryl said ;) .... boy oh boy you must have had a "vegetarian" thanksgiving Darryl.. all that work and I was to lazy to even look anything up or even attempt the full on technical route... well and you know how lousy I am at converting theoretical formula using the keyboard.. usually I attempt it and hope that people have the basic theory down mathematically and can actually translate my hieroglyphics

Hope you had a great Thanksgiving ..it actually got COLD here in FL. this year for thanksgiving..down to 45 last night!!! It actually real feels like football season at last...

Happy Turkey weekend

Jeff "I'll leave the tough stuff for the younger crowd" Trial

[Optical Plumber]

To Jeff and Darryl,

Thank you both so much for your excellent replies to my questions.

Terry
:)