Rob O. wrote: > > I'm about to venture in to this uncharted (for me) territory of > purchasing eyeglasses online. I'm tempted to try a pair of rimless - > which would also be a first for me -
For my part, I think rimless and semi-rimless glasses are an inherently bad idea. The lens is an optic and should not be used as a structure; to do so introduces incentives to use materials that are structurally good but optically lousy (like polycarbonate). > but a little nervous about the edge thickness of the lens. Polished lens edges look nice even when they're pretty thick. Any plastic lens material can have edges polished, and most online providers perform this service for no extra charge. > In my current glasses which came from a brick & mortar shop, the left > lens (-2.50 spherical, -1.50 cylindrical, & 160 axis) is about 53mm > wide & 27mm tall. At its outer-most edge, the lens is 6mm thick. I > have no idea what the lens index is specifically, although I recall at > the time of purchase, that I had to pay a bit extra and wait a couple > of days for "High Index" lens. That is not high index material-- or if so, the center thickness must be formidable. Your 'script is not strong. With a 1mm center thickness, your lenses should be quite thin if made from any material that deserves the name "high index". For what it's worth, my sister bought a comparably sized pair of so- called high index lenses from a chain optical, paid a fortune to get them, and they came out thicker than my similar prescription, similar size lenses in Zenni's default 1.57 index material. I think she got cheated, either by being charged for something she didn't get, or by the optical using a massive center thickness out of liability sensitivity. Perhaps you got frames that were too wide (or widely spaced) for your face? If your pupils were much inboard of the centers of your lenses, then the lenses' outer edges would be needlessly thick. For proper fit, the width of one lens plus the center width should equal your interpupillary distance (PD). If you really want a very lightweight and thin lens, then go for one of the truly high-index polyurethanes like Zenni's 1.67 index material. It isn't as good visually as the default 1.57 material (I have both), but it's fairly strong and shatter resistant and costs only $37 more than the regular stuff. A high index lens's optical shortcomings will be only in proportion to the power of your prescription. With your mild 'script, you might not even be able to tell a difference in visual quality. > I read something that implied that > higher index lens might be less optically-correct than standard index > lens - any truth to that? Generally speaking, the higher the refractive index of a material, the lower its Abbe number (which is a ratio of refractive index to chromatic aberration, and a good indicator of optical sharpness). The best Abbe numbers belong to crown glass, CR-39, and Trivex-- and these have the lowest refractive indices of any common lens materials. Polycarbonate has the distinction of a terrible Abbe number combined with an unremarkable refractive index. It has an index of just 1.58, but its Abbe number is even lower than that of 1.9 index glass. So it's a poor choice for lenses-- great for rimless frames, though! > And finally & more generally, I'm a new Dad and my glasses are taking > a bit more incidental abuse from my rough & tumble toddler. With that > in mind, am I just crazy for even considering rimless? Would a hinge- > less design be better or worse for enduring minor bumps? My experience with rimless glasses as a kid suggests that the first incident is likely to be their last. Frames can often be reshaped and/ or repaired (I have done a lot of both), but lenses... not so much. If you buy from an ultra-low cost provider like Zenni or Firmoo, you can offset this shortcoming by laying in a supply of replacements in advance, while still saving a bundle. There are no glasses that make you look like you're not wearing glasses. Even chunky frames can look swell if they are the right shape and size to complement your face. So in my opinion, there is little reason to mess around with flimsy, structurally compromised frames when you can get the real thing for the same price or less, and take better care of your lenses in the process. 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 -~----------~----~----~----~------~----~------~--~---
