Presbyopia
 

Presbyopia is the primary reason that nearly everyone after their mid-40's uses bifocals, reading glasses, or removes their distance glasses in order to read at a comfortable distance. Prior to the development of SRP,(Surgical Reversal of Presbyopia) external lenses such as bifocals and reading glasses were the principal alternatives available to counter the effects of presbyopia. Presbyopia is the inability to focus on near objects that requires you to wear reading glasses or bifocals. Opthamologist (eye doctors) disagree about what causes presbyopia; most believe it's caused either by stiffening of the eye's lens, continued growth of the lens or atrophy of the muscles controlling the lens, but other theories exist  as well.