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.
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.
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