Why would a 1.74 index lens be more prone to chip and be less resilient than 1.67?
My optician counselled me
I don't recommend a 1.74 index with a metal grooved frame, like the one you want from ic! Berlin, because the edge will be less resilient and be more susceptible to chipping.
Please explain like I'm 5 years old. I don't know physics or opticianry. Why's 1.74 more brittle? Why does "chipping along the groove" mean? What does "thinning" mean?
Think of lenses as having 4 levels of thinning; Basic 1.49, 1.59/1.60, 1.66/7, and 1.74.
Why would 1.74 be too fragile? Why a lot of stress on the lenses for metal groove frames like those?
1.74 will most likely chip and break especially along the edges of the groove.
My optometrist, different person, diagnosed me with nearsightedness. His prescription shows
OD –6.00 (Sphere), –1 (Cylinder).
OS –7.00, –1.
1 answer
To be optically equivalent, the lens made from lower index of refraction material needs to be thicker.
My guess is that your optician thinks the metal grooved frames you want won't work well with the outer thickness of the lenses. Note that he's not saying 1.74 index is bad, but just that it's somewhat incompatible with the particular frames you have chosen.
-7 and even -6 is rather extreme. This limits other tradeoffs.
0 comments
2 comments
It’s worth noting that there is no direct relationship between indexes of refraction (at the top of the post) and corrective diopters (at the end). The closest you come is the lens-makers formula which tells you how to take those two numbers and build a suitable lens. — dmckee about 2 months ago
@dmckee: Higher index of refraction allows for less thickness change to get the same diopter. For negative diopter, the lens will be thin in the middle and thick on the outside. -7 is extreme (to the point where I wonder if it is correct), so the edges of the lenses will be significantly thicker than the center. Frames have edge thickness limits they can handle well. At -2 diopter, for example, this might not have been a problem. — Olin Lathrop about 2 months ago