4eyesPhil wrote: > > It seems to me the higher the index of refraction the higher the > chromatic aberration ie: Snell's Law. So wouldn't the cheaper lenses > actually be of sharper focus?
There is variation in the amount of chromatic aberration intrinsic to different lens materials, and it is not simply a function of refractive index. The Abbe number represents the ratio of refractive index to chromatic aberration. Higher is better. The best Abbe numbers are in materials that have low refractive indices, e.g. crown glass, Trivex, and CR-39. But the worst Abbe number among commonly used lens materials is that of polycarbonate, which is a mid-index material. Even 1.9 index glass (the highest- index material now available) has a better Abbe number than polycarbonate. I have used all sorts of different materials in my glasses. Generally speaking, you are right-- I get the best acuity from lower index lenses. But sharpness when looking through the optical centers of the lenses is pretty good in any case-- it's off-axis sharpness that suffers most from chromatic aberration. And high-index lens material can make glasses that are easier to wear and present less of a visual disruption at their edges. Lately, I switch freely between glasses with 1.57 index and glasses with 1.67 index. They are both satisfactory, but I am really quite pleased with the visual clarity I get with the 1.57 index lenses included in Zenni's base price. I don't know what the Abbe number is, but image quality out at the edges of the lenses is better than anything else I've tried in years. Chalo --~--~---------~--~----~------------~-------~--~----~ Check us out at the oft-updated http://glassyeyes.blogspot.com! You received this message because you are subscribed to the Google Groups "GlassyEyes" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/glassyeyes?hl=en -~----------~----~----~----~------~----~------~--~---
