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Prism thinning is an optical manufacturing technique used exclusively on progressive addition lenses (PALs). The process involves grinding a specific amount of base-down prism into the bottom portion of the lens to make the lens thinner and lighter.
read more about prism thinning in progressives ...Prism thinning is necessary because progressive lenses are significantly thicker at the top due to the way they are designed. This thickness difference makes the lens heavy and unbalanced. Thinning removes weight and makes the lens center closer to the frame's geometric center.
By grinding a slight base-down prism across the entire lens, material is shaved off the top and central portion. This shaves off unnecessary weight, which makes the progressive lens much more comfortable to wear, especially in larger frames.
The prism introduced during thinning is equal in both eyes, which means it should not affect the wearer's vision. The goal is purely cosmetic and comfort-related. The slight prismatic effect is compensated for by the brain and is unnoticeable to the wearer.
The weight and balance of your backup glasses are important for comfort. Prism thinning helps make sure your progressive lenses are as light as possible. This comfort is important when your eyes need a break and you switch from feather-light contacts to your backup pair.
No, the thinning process only affects the internal structure and is not visible on the lens surface.
Most progressive lenses, especially those with high prescriptions, undergo prism thinning to balance the weight.
No, since the prism is equal in both lenses, the effect is nullified, and it does not cause the eye to misalign.
Prism thinning (yoked prism) is essential for making progressive lenses look identical in thickness. If one lens looks significantly thicker than the other, return them to the lab. Poorly calculated thinning can cause a "ground-is-rising" sensation when you walk.
Optical Laboratory Association. Surfacing and Finishing Guide (ola-labs.org). 2024.
Review of Optometry. Prism Thinning in Progressives (reviewofoptometry.com). 2023.
ABDO. Prismatic Effects of Spectacle Lenses (abdo.org.uk). 2024.
The Vision Council. Laboratory Processing Standards (thevisioncouncil.org). 2024.