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quality of vision in high rx, poly vs plastic
Patient has -5.75 OU Rx. Doesn't want to pay for high index. Doesn't want aspheric lenses or anti reflective coating. Comparing spheric poly and cr 39 plastic, poly will be thinner but plastic has a better abbe value. Since the Rx is high, the cr 39 plastic lenses will the 'coke bottle' / 'fish bowl' effect more than the poly, but how will the quality of vision compare between the two in this Rx?
Is a high index 1.67 lens always lighter than polycarbonate?
I'm confused by what specific gravity is and how that relates to the weight of a lens. I thought that the lower the specific gravity, the lighter the lens. I also thought that the thinner the lens, the lighter the lens. However, cr 39 plastic has a slightly lower specific gravity than high index 1.67, and both polycarbonate and trivex are even lower. After reading some of the things below, it makes sense that high index 1.67 is lighter than cr 39 plastic. But, what about the weight of high index 1.67 in comparison to polycarbonate and trivex? Are the three lenses usually about the same weight when comparing lower prescriptions, and high index 1.67 lighter for higher prescriptions since it will be thinner than polycarbonate and trivex?(assuming a comparison is being made of the same lens manufacturer, prescription, and frame). Reading material: Specific gravity (SG) is defined as the ratio of the density of a given material to the density of water at a specific temperature and pressure. Since lenses become lighter the lower the specific gravity value, lenses with a low specific gravity tend to have a low refractive index, and, in some cases, this necessitates thickening of the lens, resulting in increased lens weight. Also, an online article: https://www.2020mag.com/article/living-in-a-material-world