What type of modification involves removing the bottom surface of a lens?

Truncation refers to the process of removing a portion of the lens, specifically from the bottom surface. This technique is often employed to enhance the fit of the lens or to improve comfort for the wearer. Truncating a lens can also help in reducing the overall thickness or weight of the lens, which is particularly beneficial for certain prescription types or designs.

In contrast, polishing involves smoothing the surface of a lens to enhance clarity and reduce optical imperfections. Edge shaping refers to altering the edges of a lens to create a specific shape or contour without removing material from the bottom surface. Coating pertains to applying a layer to the lens to enhance features such as anti-reflective properties or UV protection, which does not involve altering the physical surface of the lens itself. Each of these processes serves different purposes in lens manufacturing and fitting, highlighting the importance of understanding their specific definitions and applications.