So if light travels SLOWER through a High-Index lens, does that mean that the visual acuity is not that great?
So if light travels SLOWER through a High-Index lens, does that mean that the visual acuity is not that great?
Relative to a lower index, yes.
The VA shouldn't be any different thru the optical center, if you are correctable to 20/20 you should be able to see 20/20 thru a High index material. If you are referring to the ABBE value, yes high index materials have lower values. What some people complain about are peripheral and lateral distortion after leaving the optical center of their lens.
Paul nails it.
My answer was incomplete and shouldn't have been written, sorry!
There are other factors, for example, a both 1.71 and 1.74 index have a higher ABBE than 1.67, and Poly at 1.59 is worst of all. A given material can disperse light a rate differently than its index, so lenses in GENERAL will have a lower ABBE relative to their index, but its not an exact ratio.
A greater value for refractive index means that the material is more efficient in bending (refracting) light, that is, less curve is required for the same dioptric value. The result is a thinner lens.Originally Posted by spex911
However, in general, as the refractive index increases, so does dispersion, creating higher levels the Lateral Chromatic Aberation or LCA, which can result in blur and/or color fringing.
http://www.opticampus.com/cecourse.p...ic_aberration/
Thinner lenses, due to changes in refractive index alone, are not necessarily lighter weight lenses though. In general, most medium to high index lenses weigh about the same, plus or minus a few percent. The reason is that in general, as the refractive index increases, so does the specific gravity. The thinner lens has less volume, but the increase in density makes it a wash. Don't tell your clients that high index lenses are lighter in weight than lower index lenses, unless the low index lens is CR39 or glass.
Hope this helps,
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
I believe the OP is referring to the fact that light travels slower through a higher index lens.
The Abbe value is only a measure of dispersion and chromatic aberration of light through a given medium, but it is not a direct relationship to the actual speed that photons travel through that medium.
That's one of the selling points of trivex if I'm not mistaken, that it is able to let light travel faster through it, and it also has a higher percent transmittance than any other material of similar index. meaning it in theory should allow more light to enter the eye, thereby increasing visual acuity(in low light situations).
If I am off base, or wrong, please feel free to ignore/correct me.
I am not an optics/physics major(Ochem is more fun).
The Abbe is a direct relationship to speed of photons. Higher indexes slow light more, which bends it more, which causes the different wavelengths to diverge more, which is chromatic aberration. However as was astutely pointed out, chromatic aberration occurs only away from the optical center so will not have any impact on acuity through the optical center. Peripheral acuity yes, but not simply due to the photons being "slow", it's because of the loss of focus from diverging wavelengths that result from the slowing/refracting process.
The selling point is the reduced chromatic aberration and therefore clearer peripheral vision. It may be slightly clearer (higher transmittance) than high-indexes, but no clearer than poly or CR-39. The "allow more light to enter the eye, thereby increasing visual acuity" concept is much better address by AR.
Re the OP, just because light moves slower through different medium reduce acuity, it just means your view of the world is slightly behind everyone else's! Hey that's renewed marketing for CR-39 and glass: "Don't see the the world as it was, see it NOW! Don't fall behind with expensive, slow, high-index lenses! Increased your reaction time by seeing REAL TIME with super fast CR-39!!"
If I am not mistaken, the abbe value is not an absolute direct relation to the speed of a photon traveling through a medium(index of refraction).
Other wise how would we account for crown glass having a higher abbe value than CR-39, while having a higher index of refraction.
Like-wise with 1.67 vs 1.74.
But I agree that VA should be relatively the same, given that when the index of refraction goes up, lens thickness decreases(normally).
The snellen acuity may remain the same however the grating acuity would definitely be effected by the abbe of a lens material. Since high index lenses generally experience a reduction in abbe that is proportional to the increase in index (most cases not an absolute), it is sufficient to say that high index lenses will have an effect on the grating acuity of the eye or decrease the contrast sensitivity of the eye.
Example image below of a grating, wearing a high index lens move your head from side to side while viewing the image and watch the contrast increase and decrease.
Right, It's the difference in speed through the medium, at different frequencies. Lenses are not more dispersive because they have a higher index of refraction per se, but probably because we don't know how to make a very high refractive index material with low dispersion, and still be serviceable as an ophthalmic lens.
That, and impact/chip resistance. And it's not always off-axis blur, there can be chromatic aberration on-axis when the OC is not aligned with the primary gaze, typical with high drop PALs and/or high panto tilt values, and when there is prescribed prism.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
I'm under the impression that:
dispersion/chromatic aberration
reflectance
refraction
are all directly proportional to index of refraction. The way to answer that would be to trot out the formulas and look. Alas, I'm too lazy.
HC, I don't see why any measurement of visual acuity, including contract sensitivity, would be differentially affected through optical centers. Yes, though, off center, such as through the bottom of a PAL.
Reflectance and refraction.
R = (n - 1)2/(n + 1)2 * 100%
http://hyperphysics.phy-astr.gsu.edu...pt/lenmak.html
http://hyperphysics.phy-astr.gsu.edu...ispersion.html
Last edited by Robert Martellaro; 07-24-2014 at 09:09 AM. Reason: damn markup language
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Ok, thanks.
All we see is constants and index of refraction, so yeah, that one is inextricably linked.
As Mr. Scott would say to Captain Kirk: "Ya canna change..."
I looked at that dispersion link, and there was nothing in it but constants and indices of refraction for different wavelengths.
To me, it's a surprise that index and dispersion aren't totally linearly related, but we do have evidence that some polymers perform differently, as though the function line is "bumpier" than expected. I wish I understood why that's true.
Anomalous dispersive materials lie off the the linear slope relationship between index and dispersion. Lanthanal glasses are of particular note here.
B
We would probably need to take it to to the atomic level, where this kid gets off the bus.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
If we consider the very real scenario where no subject can strictly focus on the single ray entering the eye through the optical center of any lens and instead compare the size of the airy disc with an aperture the size of an average pupil, then unfortunately the quality of the visual acuity does suffer in hi-x or more accurately low abbé lenses. (Beam tracing vs ray tracing)
We could go as far as comparing the reduction in transmission through additional lens thickness compared to a low abbé lens to determine a threshold for when the lower abbé material exhibits better overall visual quality, but non of this matters when a client sits across a dispensing table with a thick lenses wondering why you as a dispenser are subjecting them to the reduced cosmetics. I think vanity will win more than 90% of this argument.
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