Bifocal lenses are used for individuals who have found it difficult to see things at multiple distances. For example, someone wearing single vision glasses for distance might notice the words begin to blur while they are reading a book. The eye doctor may determine that this person needs glasses with two separate powers to help them see at two separate distances. The doctor will prescribe lenses that put their distance prescription in the top portion of the glasses and the reading prescription in the bottom part of the glasses. Bifocal lenses usually have a visible line on the lens where the reading portion begins.