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Thread: Periphreal Distortion in Pt. Switching from Trivex to Poly

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    Periphreal Distortion in Pt. Switching from Trivex to Poly

    Have a low-power SV myope (-1.50 OU with -0.50 cyl OD) who recently made the switch from trivex to poly for financial reasons, and who is now complaining about periphreal distortion, especially when he turns his head-- his description sounds a bit like complaints from new PAL wearers. I'm guessing that it's chroma from the drop in Abbe quality, but, just wanted to bounce it off a few of you before I spoke with him about it. He's not complaining about any color distortion or rainbowing effects, which is leading me to think it might be an issue with the lenses, but I didn't want to send the guy down the remake-and-let's-see hole if it can be avoided. Any other thoughts? And if it is an abbe issue, are there any reccomendations for helping the guy adapt? He put forth the effort of wearing them for two weeks before coming back, so he is trying to get used to them.

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    Quote Originally Posted by Browman View Post
    Have a low-power SV myope (-1.50 OU with -0.50 cyl OD) who recently made the switch from trivex to poly for financial reasons, and who is now complaining about periphreal distortion, especially when he turns his head-- his description sounds a bit like complaints from new PAL wearers. I'm guessing that it's chroma from the drop in Abbe quality, but, just wanted to bounce it off a few of you before I spoke with him about it. He's not complaining about any color distortion or rainbowing effects, which is leading me to think it might be an issue with the lenses, but I didn't want to send the guy down the remake-and-let's-see hole if it can be avoided. Any other thoughts? And if it is an abbe issue, are there any reccomendations for helping the guy adapt? He put forth the effort of wearing them for two weeks before coming back, so he is trying to get used to them.
    Did you try reducing the vertex distance if possible? Is he private pay? If so, I would switch him to trivex at no cost, will save you a headache for negligible cost difference. I would also compare the size of his old v.s. new lenses. If his old frame was small and new frame is big, that would explain it too.

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    it could also be a switch from aspheric / digital aspheric to a spherical poly lens also. Also the OC, PDs, RX or especially lens size or vertex like Cowboy said.

    Thats a very light script for ABBE to be an issue imho

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    OptiBoard Professional Randle Tibbs, ABOM's Avatar
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    The farther your patient views from the OC the greater the chromatic distortions will be. Tallboy is correct, if the trivex is in an aspheric design and the poly lens provided is in a spherical design, it could also lead to his complaints.

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    Most likely, this was FSV. Most Labs stock spherical FSVs for this RX.

    To further complicate matters, FSV poly base curves vary from MFR to MFR. GENTEX is steeper than Vision Ease, which is steeper than Zeiss. FSV Trivex is consistent. Not sure about third party and Asian vendors.

    I agree with others regarding asphericity, vertex and CA due to ABBE, however, many can switch between the two with this RX, but the BC also needs to be similar.

    You might also consider CR39 and comping the UV, if cost is an issue. This is a fairly minimal RX.

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    Eyes eastward... Uilleann's Avatar
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    Repeat after me: ABBE. IS. NOT. THE. PROBLEM.

    We really need to stop perpetuating the myth folks. Particularly in a case like this, with such a low power. We have got to STOP blaming the material. Keep looking...elsewhere.

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    Chromatic aberration is not a myth, but is unlikely to be a problem when there's not sufficient power to induce enough prism to cause off-axis blur. The only thing left to consider is oblique astigmatism- if you pick the wrong base curve on an aspheric semi-finished lens, let's say a +2 base optimized for powers in the -4 to -6 range, we could easily see .50 D to .75 D of OA, enough to cause blur 30 degrees off-axis. About the same on a free-form platform, although there should be stricter controls in a free-form lab. Try a finished uncut from a reliable manufacturer, where the design is biased towards best form optics instead of best profile/thickness.

    Hope this helps,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    Master Jedi King of the Lab's Avatar
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    Sounds like a sensitive flower to me. Any info on the old frame vs new frame? Position of wear I assume has already been considered?
    Erik Zuniga, ABOC.

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    Eyes eastward... Uilleann's Avatar
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    Quote Originally Posted by Robert Martellaro View Post
    Chromatic aberration is not a myth, but is unlikely to be a problem when there's not sufficient power to induce enough prism to cause off-axis blur....
    This is precisely the point however. In the powers that poly is commonly used, CA is simply a non-factor for 99.999% of human beings. But it is almost *always* the very *first* thing to be blamed.

    It's lazy, completely scientifically inaccurate, and perpetuates a myth that poly is an undesirable/unwearable material - for reasons that aren't based in fact. Dispensers and doctors alike need to lay off that one note drum.

    With that said, I agree 100% that other factors including BC and OA may well be part of the above issue. In addition, cases like these can also be triggered by the fact that a patent hates a new frame style, or a spouse hates a new eyeglass bill, or that the pt may have just had a relative or friend pass away. The list can be very long. But Abbe should almost NEVER be included on it in cases like these.

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    Mild Rx, don't mind poly or base curve. Fix the simple things first. Panto and face wrap. Doesn't cost a penny. Then contemplate remakes.

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    Rising Star McAnerin's Avatar
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    Everyone here is right.

    But still...
    -Poly is the best substrate for coatings.
    -Poly is extremely scratch resistant.
    -Poly is extremely impact resistant.
    -Poly is unparalleled in clarity.
    -Poly is much lighter than traditional crown glass.
    Like poly, you can trust me about 40% of the time.

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    Quote Originally Posted by Browman View Post
    Have a low-power SV myope (-1.50 OU with -0.50 cyl OD) who recently made the switch from trivex to poly for financial reasons, and who is now complaining about periphreal distortion, especially when he turns his head-- his description sounds a bit like complaints from new PAL wearers. I'm guessing that it's chroma from the drop in Abbe quality, but, just wanted to bounce it off a few of you before I spoke with him about it. He's not complaining about any color distortion or rainbowing effects, which is leading me to think it might be an issue with the lenses, but I didn't want to send the guy down the remake-and-let's-see hole if it can be avoided. Any other thoughts? And if it is an abbe issue, are there any reccomendations for helping the guy adapt? He put forth the effort of wearing them for two weeks before coming back, so he is trying to get used to them.
    More information is needed.
    Let us assume the Trivex and Polycarbonate were on the same base curve, they were not rotationally symmetrical aspheric lenses or free form lenses, and all other PoW factors were the same.

    Pantoscopic Tilt = 9 degress
    Vertex = 12 mm
    Wrap = 6 degrees

    The effective Rx due to PoW is -1.55 -0.16 X 125 for both materials

    CR-39 produces 0.05 D of oblique astigmatic error with 0.03 D of TCA and Polycarbonate produces 0.06 D of oblique astigmatic error and 0.07 D of TCA, these calculations assumed a 6.00 D BC
    TCA cut off is around 0.10 D, so neither produce enough to cause concern

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    Has there been a reduction in the total minus power? Or is the Rx unchanged?

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    Damn. Sorry it's been a few days since I got back to this. Been busy.

    So to hit a few points--

    1) Patient and spouse love the new frame and the pt. is comfortable with it. His last frame was a 55 eye, 15 DBL; new frame is a 52 with a 22 DBL. Pt. has a larger head and had chosen the last pair based on fit rather than aesthetics; the new frame has his eye centered much better than the last, which almost looked like someone had RXed a visor.
    2) Insurance job, so, changing material becomes iffy. Also has to be lab finished, so, I'd have to check with them to see if they're doing surfaced or FSV. Has a high-quality AR (Zeiss) so I'm ruling that out as a potential culprit.
    3) Thanks to the OD, CR-39 isn't an option, as she tells every patient who comes through the door that CR39 lenses will blind them in the event of a car crash because the lens will "explode" into their eyes. After hearing that from the doctor there's no way an optician is talking the pt. out of an impact resistant material.

    I've sent the lenses in for a remake just to assuage the patient; once they come back, I'm also taking care of vertex, etc. Interested to see what happens!

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    Eyes eastward... Uilleann's Avatar
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    One other aspect to touch on - as these are Zeiss lenses. Were they digitally generated? For the price, one would expect all Zeiss lenses to be highly accurate out of the gate. But reality is, that there are humans pushing buttons all along the chain, and sometimes things get messed up. Particularly with digital generation, perhaps the machine was out of calibration and induced distortion you wouldn't easily pick up with a lensometer. It doesn't happen all the time, but it does happen, and more than once a year or so.

    With luck, your remake will help solve the issue if that's a possible culprit.

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    What's up? drk's Avatar
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    Just wear them for a while.

    You should have seen me when I switched to Auto II PALs in 1.67 with my -6.50/+2.00. I thought I was gonna die. Now I love them (except night driving).

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