I'm not so sure about that Ira. I've got five pairs of glasses with 1.67 index lenses of nearly identical width (+/- 1.0mm). Each one was made at a different lab - two at a B&M lab in Los Angeles, the others were from Zenni, Coastal Contacts and another Chinese retailer. Zenni's are by far the thickest - 2.2mm at the edge, 4.96mm at the center, in a full rim metal frame. The rimless lenses are about 2mm at the edges and vary from 3.6mm to 3.9mm at the center. As a reference, I also have a polycarbonate rimless pair from LBW Eyewear, with lens edges of 2.55mm and a center thickness of 5.32mm. And, my most recent pair of rimless are 1.74 index (Coastal Contacts) are 2.0mm thin, 3.4mm thick.
I've never seen opticians have mockups of the exact same frame and prescription but in different index lenses - this surely cannot be due to cost, as we know lenses cost very little to make - I suspect it's more to protect the labs from having to be consistent. It looks like labs use different size diameter blanks (65, 70, 75mm) depending on the application and some do additional surfacing to try and thin the lens edges. These factors are more likely to blame for the differences in thicknesses. Further to the OP point of US standards being more safety-biased than everyone else, this is a well known fact, google 'ball drop test' (http://www.ecpmag.com/1webmagazine/2008/01jan/content/FDA- impact-resistance.asp). Getting glass lenses made in the US is extremely difficult - many opticians gasp at the thought of crown glass lenses - they've been very well indoctrinated by the industry that they're not safe. Many other retailers won't dispense rimless frames in anything other than Poly - even though we know very well that MR-10 1.67 Seiko is just as crack/shatter resistant. Just speak with a US lab technician, they'll tell you that they won't go less than 1.5mm or 2.0mm for a center thickness - when in Germany and Italy for example, they regularly machine to 1.0mm CT - the UK might have similar standards. There are clear differences in lab standards between the US and even Canada - have a look at the OptiBoards forums and it's well covered. You are right that optical center can have a significant impact as well and that may very well be a factor here. J. Evan On Aug 5, 4:53 pm, Ira Mitchell <[email protected]> wrote: > I'd assume the laws of physics are much more important in this > instance than those of the United States. > > Sounds to me like either you didn't get 1.67 or the optical center of > the lens is further from that "thick edge". > > - Ira > > On Aug 4, 2:28 pm, Jon Edwards <[email protected]> wrote: > > > > > I have just moved to the US, and bought myself new glasses at > > Lenscrafters. To my surprise, with an identical prescription (-7.00), > > identical high-index (1.67) and identical frame size and shape, my US > > glasses are a good two millimetres thicker at the thickest point. > > > I'm told that this is because safety standards vary around the world > > regarding lens cutting, and US lens manufacturers have different > > standards (presumably because the US is so litigious). Apparently the > > UK and Hong Kong both cut thinner lenses. > > > That being so, any insight as to where the lenses are cut for online > > stores, in general or specifically? And under what standard? > > > Any information at all very welcome! > > > Thanks > > > Jon --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
