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?

ANSWER

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

You can leave a response, or trackback from your own site.