One of the main problems with bifocal and trifocal lenses is the issue of eye fatigue. It can be difficult to switch from one focusing power to another. Your eyes can tire, which can even lead to a headache, sore neck and sore back.
A variation of bifocals and trifocals is the no-line lens or progressive lens. No-lines provide a smooth transition from focusing on nearby to distant objects because they do not have a distinct line separating the focusing powers. Instead, a gradual change in power allows the wearer to focus on objects at all distances. Distant objects are viewed through the upper portion of the lens, while near objects are viewed through the middle or lower portion of the lens.