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Possible Problems with High Index Lenses
Guillaume wrote:
I recently bought a high quality plastic frame outfitted with Carl Zeiss 1.67 high index lenses w/Carat Advantage coating. My prescription is -3.25 LE and -2.75 RE with no astigmatism/other problems. Looking straight without moving is great but the image gets distorted if I move my head up/down and left/right (aberrations?). When I move my head up/down the image gets ''compressed'' and when I move left/right it ''tips'' or gets bent. It's a bit hard to explain I guess. I will consult the seller but I'd like to get your opinion or what could cause these visual disturbances (wrong PD? Lenses too tight in frame? etc.) Thank you very much for your help, Guillaume.
Dr Langis Michaud's response:
Dear Guillaume
The first thing to consider is the high index of the glasses you got. Usually, this type of material is selected when the prescription is very high and we need to make the lenses thinner. This is not the case with your refractive error. Perhaps the frame you have selected dictated that need but I cannot really explain why it was provided to you. High index lenses are made thinner but they also compromise some elements of their optical function. For example, the movement and distortion you feel are directly linked with the behaviour of this material, outside of the optical center. If your frame is quite large, this is worst. You are right to say also that some mis-manufacturing process can occur: a tight lens will create distortion and will lead to the symptoms that you have. It would be surprising however that this affects both lenses at the same time.
My recommendation: go back to your provider and ask to redo the job with a regular index – if this is not possible due to frame constraints, select another frame- the person who guided you did not do a good job if the frame was selected too wide for your needs.
Good luck
Dr Langis Michaud, OD, MSc, FAAO (Dipl)
Associate Professor – U de M
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Re: Distortion and Barrel Effect
Breen wrote:
I am a myopic mid-50s adult with extremely good peripheral vision. As a result, I am very sensitive to distortion of any form (the barrel effect), and am aware at all times of the edges of my glasses, even though they are attached to the bridge and temples only. I also have a second pair of single vision glasses with a field of focus optimized for computer use. Progressive lenses are out of the question. My previous single vision glasses were Nikon high refractive index lenses (Nikon 5, with RI 1.74), and I was very happy with the level of distortion, which while always present, was tolerable. My new prescription (OD/OS -3.25 spherical only, 0 cyl.) resulted in an opportunity for me to try out the Nikon SeeMax lenses, which promise minimal distortion. However, the result is exactly opposite from what I expected, and exhibit far more curvature distortion than my old lenses. My previous prescription was OS/OD -2.25 spherical, which is a significant difference. However, when I look at the details on the new glasses receipt, I see they used the 1.6, and not the higher 1.74 (or even 1.67) refractive index lenses. This was a surprise to me, as I assumed that they would order the same RI as the old ones, and I will be getting them to replace them with the thinner 1.74 lenses to minimize the visibility of the edges of the lenses (due to extremely good peripheral vision). Could I expect less curvature distortion with the higher refractive index of 1.74, vs. the 1.6 that I'm unhappy with? What if anything, does the base curve have to do with distortion, and how is this related to the refractive index for the lenses in question? Also, are there other lenses which might be more suitable for me than the Nikon SeeMax to minimize peripheral distortion?
Dr Henry Smit's response:
Dear Breen From your letter it appears that you are a very “sensitive observer” – someone who notices distortions that the average person would not detect. The barrel effect that you mention tends to be more noticeable with an increase in the size of the lens that you are looking through. If there is more peripheral lens to look through, you will be exposed to more peripheral distortion. Although you did not mention lens size in your letter, if your new lenses are larger than your old pair, it may be a factor in the distortion you are experiencing. Typically, as the index of the lens increases, there is an increase in the amount of radial astigmatism (another form of distortion) in the lenses. However, the customized lens surfacing techniques such as those used with the Nikon SeeMax have typically made great improvements in reducing the level of the other lens distortions (such as barrel distortion) experienced by the wearer. However, a -3.25 lens will induce more barrel astigmatism than an equivalent sized -2.25 lens made of the same material. A 1.74 index lens at -2.25D would be flatter and thinner than a 1.6 index lens at -3.25D and this difference, in your case, may also be contributing to the distortion you are noticing. The Nikon SeeMax is a well designed lens with a good track record. There are other lens options from different manufacturers that also perform well. Unfortunately, there is very little reliable independent data available that compares lens performance from different manufacturers. Most of the information available is self-generated by the lens manufacturers, and to no one’s surprise, each manufacturer appears to claim that their own lens performs best. Hard data aside, the other factor that the lens manufacturers cannot control is the visual perception of the patient wearing the lenses. It has been my experience that patients with same the very similar lens prescription, visual demands and frame size still express different preferences in their lens design choices. Based on your apparent sensitivity to distortion, it may be advisable to stick with a frame and lens design that you have successfully worn in the past. Boring, but it sometimes does the trick when all else fails! Dr. Smit
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Re: Serious Distortion with New Lenses
Mike wrote:
Hi, I recently had my prescription updated and went to get a new pair of glasses but ran into some visual distortion issues. My normal optometrist (that I have been seeing since I was 10, I am 27 now) was not available at the time, so I made use of the in-house one where I was getting my glasses. The prescription [I don't have it] (which was later checked by my normal optometrist) was accurate, save for the prism. For whatever reason, She could not see it while taking the measurements. So she added the prism from my glasses that I was wearing at the time (a 1 B0 prism). The optician did the glasses. When I tried them I noticed a significant distortion of the ground immediately in front of me, it was like I was walking up a 3' hill all the time. This was looking ahead normally while walking. It was also like they were magnifying the image as well, as if I looked straight down, my body/legs appeared to be about 2' tall (I'm 6'). I made a point of wearing them for a few days to see if it was an adaptation issue but there was not noticeable improvement. At that point I went back to the opticians and they rechecked everything PD, prescription, as well as positioning of the frames on my face to no avail. They suggested that I go to my normal optometrist to double check the prescription. Which I did and ended up with: OD -800 -075 175 1.5 B0 OS -650 -050 008 1.5 B0 I took this back to the opticians and they re-did the lenses with the new prism. These were a no go as well. Same visual distortion. Again, they checked the lenses, position etc. All seemed ok. Though they noted that the base curve of the new lenses was not the same as my old ones. So they suggested that they re-grind the lenses with the old base curve to see if that would fix the situation. While it did improve the distortion it was no longer a 3' hill immediately in front of me, there was still a significant rise in the ground probably from 6 to 10' away, I would have to guess that it looked like it was about 1'-2' rise, though it was smooth not abrupt, kinda like a pronounced crown in the road. At that point I had given up on the glasses as the opticians could not provide any answers or other things to try, other then to check the prescription with my optometrist again. Which I did and ended up with the most recent prescription of: OD -775 -075 175 1.5 B0 OS -675 -050 006 1.5 B0 I have not had this made into a set of glasses yet, as I don't know what was causing the problem and I am a little leery of going through this again considering the cost of the lenses alone usually runs about $300+. The lenses they were using above were Zeiss 1.67 RI, their lab was using Essilor Kappa. The frame size was 49-19, Oakley Rotor S. My old glasses (which I am still wearing) were from Lensecrafters, the lenses are Featherweights (polycarb 1.54 RI?). Frame size 46-18, similar lens shape to the Oakleys, rounded rectangle. Old glasses prescription is: OD -750 -050 180 1.0 B0 OS -625 -050 180 1.0 B0 Any ideas what was happening? Thanks for your time.
Dr Henry Smit's response:
Mike Dear Mike I will do my best to offer some suggestions based on the information that you have given me. For starters, yours is a fairly high prescription, and high prescriptions create more distortions than lenses made in a lower prescription. Secondly, I suspect that you are a sensitive observer and that you may be bothered by induced distortions that might otherwise go unnoticed by the average observer. It appears that your new prescription is a little bit stronger, that you are wearing a slightly larger frame, that the amount of “base in” prism has increased slightly and that the axis of the astigmatism has changed slightly (going from exactly horizontal to being tipped up slightly at the outside of the frame). Each of these changes, by themselves, would probably not cause a great change in visual perception. However, these four small changes, in combination, may be enough for a sensitive observer to notice. In addition, it is possible that the “wrap” of the new frame is different that the vertical centration of the lenses may be different from what you are used to wearing. These factors could make your adaption to the new glasses a little more difficult. Fortunately, most patients do eventually adapt to changes in prescription and lens design. If you have worn the glasses for a period of a few weeks and do not experience any improvement in your symptoms, you may want to try to go back to a smaller frame. As a last resort, you may also want to go back to the polycarbonate lenses, but any improvement in spatial perception you might gain may also be offset by the generally lower optical quality that polycarbonate lenses provide away from their optical centers. I wish you luck. It sounds that you and your different care providers are doing their best to make things work for you. Hopefully with a little more perseverance you will be successful. Best of luck Dr. Smit
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