eyeglasses with smallest chromatic abberation

mike1127

I have high-power eyeglasses (about -10 diopters in one eye, and -12 in the other). I have always been bothered by the chromatic abberation--- almost any visual object (reading text, reading music, street signs, traffic lights, etc) is clear only in the center of my lens, and gets increasingly blurry as one gets away from the center.

I've had my lenses always made from a higher-index plastic.. not the standard cheap plastic. Usually I ask for 1.56. This makes the lenses thinner and lighter. However it has been my observation that higher index plastic has more chromatic aberration than lower index

I was reading the Wikipedia article on "corrective lenses" (The forum won't let me link to it, but you can find it easily)

It appears that the chromatic aberration depends on something they call the ABBE value. This gets me wondering, is there a way I could get lenses that are both thinner and have less chromatic aberration, by choosing a plastic with a high ABBE value (high is good)? For instance, in the chart in that article, I see that the material I usually use, 1.56 plastic, has a low ABBE value (bad), while an even higher index material (1.60) has a higher ABBE value.

The question is, does the amount of distortion depend only on the ABBE value, or does it also depend on the refractive index at the same time? Can I predict that going to 1.60 plastic will increase/decrease the amount of distortion? Or does it depend on other factors, like lens power, base curve, optical center position, lens coatings, etc?

I can't use contacts because I have prism in my lenses (2 diopters inward in each lens, and 1 diopter up/down skew). Also I have chronic eye irritation from Grave's disease so I can't tolerate putting anything in my eye.

Low-Q

When you look in other directions but straight forward, your iris is not in the center of the optics and you look in a direction that is not optimal for the optics to handle. This will result in an optic that is not longer aligned correctly to your eye.

Also, optics are more or less "directive" in terms that they cannot correctly focus if the source you're looking at is coming from an angle with respect to the eye glasses and the eye.
An example is the experiment with a magnifying glass that is focusing the light from the sun. The optics will make a crisp and clear projection of the sun on a surface when the magnifying glass is aligned angular to the sun light.
If you turn the magnifying glass slightly, the projection will be blurred and distorted because the sun light isn't longer angular to the optics. The stronger the magnifying glass, the more noticable this effect will be.
The reason is that the relationship between the curves on the surfaces on the magnifying glass is not equal towards the perifery of the glass as it is in the center. So the optical properties will change depending on the lights angle of attack.

Complex optics with several lenses will partially correct this. As in a video projector, you have a given extent of the image source. To project a good image on the canvas at the wall, there must be optical correction. That correction cannot be done properly with single optics.

Acrylic lenses will in addition expand and contract depending on high or low temperature, changing the focal length, which in turn makes the optics even worse to look through.

Eye glasses made of glass is heavy, but is not as much affected by temperature as plastic/acrylic optics are.

Vidar

rcgldr

Any chance that a combination of contact lenses and glasses could help here? I assume that contacts alone can't produce -10 to -12 correction.

Low-Q

Contact lenses will always follow the eye, so if you combine contact lenses with glasses, the glasses can be made less strong, therfor less distortion.
So the answer should be yes. A combination would help here.

Vidar

mike1127

Thanks everyone. This is great information. Unfortunately I can't use contacts due to chronic eye irritation that I have.

I still would like to know if there is some way to predict distortion -- I need to choose which plastic to use to make my next glasses -- for instance, 1.56 vs. 1.60 (index) -- these two materials not only have a different index but also have different ABBE values. Since lenses are hugely expensive in my prescription (and I need AR coats also for the glasses to be at all practical) it would be nice if I could predict the result.

Andy Resnick

There's some bad information in this thread. To your question- the Abbe value of a material is a measure of the dispersion: how the refractive index varies with wavelength. Specifically, the Abbe value is calculated for the index of refraction at a blue-green, yellow-red, and red wavelength (486.1 nm, 587.6 nm and 656.3 nm)

http://en.wikipedia.org/wiki/Abbe_number

Different materials have different Abbe values. Lenses are corrected for chromatic aberration by using two or more different materials (an achromatic doublet, for example, uses a 'crown' and a 'flint' glass). An achromatic eyeglass would likewise require multiple materials.

Does this help?

mike1127

Well, I'm trying to determine what index plastic I should use for my next lens.

There seems to be two kinds of distortion that are most obvious, as seen by me when I look away from the center of the lens.

One of them is chromatic aberration -- I believe this is chromatic aberration because it causes colored fringes around edges. This is very significant -- text is illegible at the edges of my glasses and any edges have wide color fringes.

The second is the effective change in power when looking away from the center of the lens. I can tell this is going on because it takes me a moment to refocus my eyes when I look away from the center. I *can* get things to be clear again after I refocus (as clear as possible given the chr. ab. anyway).

So, should I get 1.56 plastic or 1.60? 1.60 has a higher ABBE value. Does that mean it will have less color fringing? Is it that simple? Or does the index of refraction influence the color fringing also, such that a higher index means more fringing?

How about the change in effective power? A 1.60 lens will be thinner and flatter, which may change the effective angle of the lens surface when looking away from the center.

Andy Resnick

High-refractive index materials are usually chosen to reduce the weight and thickness of the lens. Monochromatic aberrations (you describe something similar to 'field curvature') are a function of the surface shapes. Chromatic aberrations cannot be corrected when using a single material.

mike1127

I'm aware that chromatic aberration can't be corrected with a single material, but I would like to find a material that minimizes the color fringing and I would like to know how to predict that, knowing the ABBE value and the index. In particular I'm wondering if it's something that depends solely on the ABBE value or if it depends on the index too.

Drakkith

Higher ABBE numbers mean less dispersion and the ABBE number generally increases as refractive index drops. The amount of dispersion between a 1.56 and a 1.6 material is extremely insignificant and you will most likely not notice any difference. I doubt you would notice the difference between two materials unless the ABBE number difference between the two was at least 20 or so, and even then it would still probably be too severe.

As for predicting the ABBE number, I think that requires actually measuring the dispersion and refractive index of the glass. But there is a chart for it here: http://en.wikipedia.org/wiki/Abbe_number

Andy Resnick

There's some information here that may be of interest:

http://www.mastereyeassociates.com/e...ens-materials/

mike1127

I *do* know the ABBE value and the index for each type of plastic. Again, my question is whether the color fringing depends on the ABBE value alone or whether it includes the index.

I assure you the differences between materials are extremely obvious. The question is, how much of my visual field is clear? 1.54 and 1.56 materials have different ABBE values. 1.54 has much less color fringing but is significantly heavier. If you explain the difference in terms of how much of my visual field is clear (for instance, how much is usuable for reading), 1.54 has maybe twice the usable area than 1.56. But it is too heavy to tolerate, so I have gone with 1.56 in the past.

What I notice is that the 1.60 plastic has a higher ABBE value (significantly). I want to try to predict whether it is worth making the lens out of 1.60 and whether the color fringing will be less. The ABBE value is higher, but does the higher index negatively affect color fringing?

So to make this very clear: I want to try to predict whether 1.60 has less color fringing, and I'm asking a technical question which is basically does the color fringing depend on ABBE value *alone* or does it include index?

Mike

mike1127

Thanks for finding it but this doesn't help. It gives little information about high-index plastics, the very plastics that I want to know about. I'm aware that Trivex has just about the highest ABBE value of any plastic but has too low an index to be useful for me (would make a very heavy/thick lens).

This link has more specific information: it gives the ABBE value of each plastic type, but my question still remains: how to predict the width of the color fringing.

(it won't let me link because I have less than 10 posts, but type "corrective lenses" into google and go to the wikipedia article)

mike1127

a post to reach 10

mike1127

Here's the wikipedia article
http://en.wikipedia.org/wiki/Corrective_lens

jtbell

Just to nit-pick a bit, it's not ABBE (which looks like an acronym), but Abbe, the name of a physicist who was well-known for his work in optics.

http://en.wikipedia.org/wiki/Ernst_Abbe

Drakkith

The Abbe value is the measure of dispersion. The higher the number the less the dispersion so the less color fringing. It already includes the refractive index of the material in the number since it is an actual measurement of the dispersion between 3 different wavelengths of light. So two lenses with different refractive indexes but the same Abbe numbers will have equal dispersion and color fringing. However, the difference in refractive indexes will lead to different shapes for the lenses which may lead to more/less color fringing depending on the shape of the lens. Given that your lens with the higher refractive index also has the higher Abbe number it should require less curvature to achieve the desired correction and suffer less dispersion and color fringing.

Note that my knowledge in optics is purely due to my own personal research stemming from my hobby in astrophotography, so I cannot guarantee that I actually know anything.