If you experience diminished ability to naturally focus your eyes both near and far, lined or round multifocal eyeglass lenses (bifocal and trifocals) containing two or more distinct lens powers may help you see more efficiently at multiple distances. However, you should see our eye doctor to diagnosis the cause of your diminished ability so we may ascertain the correct reason and solution for any visual impairment.
"Bifocal" means "two focuses" literally. Multifocal lenses are eyeglass lenses that enable you to see clearly at two different distinct distances while only using one lens. Benjamin Franklin has been credited with inventing the first bifocal lens by cutting two lenses in half from two different lens prescriptions and gluing them together. The top half was used to see greater distances while the lower half assisted greatly with viewing objects and text within arm's reach.
Bifocal glasses are used for many reasons, but the most common reason is Presbyopia. Presbyopia is a vision condition in which the crystalline lens within the eye looses its flexibility thus hampering the ability to focus. One may need a multifocal lens simply due to age. On an average, once you hit 40 you may became a good candidate for these lenses, although you may require multifocal lenses at a much earlier or later age and for other reasons as well. Always get your eyes checked for vision and eye health reasons.