Bifocal and progressive lenses are both used to correct presbyopia, but they differ in how they provide vision correction. Bifocal lenses, made of 1.56 index material, have a visible line that separates two distinct areas for distance and near vision. However, it's important to note that these lenses cannot be inserted into rimless frames because the drilling process for rimless frames can weaken the lens material around the edges, increasing the risk of cracking or breaking. On the other hand, progressive lenses offer a seamless transition between different lens powers without any visible lines. Only the 1.61 MR-8 and 1.67 MR-7 resin options are suitable for rimless frames, as they are specifically designed to withstand the drilling of holes into the outer edges of the lenses. Therefore, if you have rimless frames and require multifocal correction, it is recommended to choose progressive lenses with either 1.61 MR-8 or 1.67 MR-7 lens index, as they can endure the necessary drilling without compromising the structural integrity of the lenses.