During the early and middle years of life, the lens of the eye provides for the capability to focus both near and distant images. To accomplish this feat, the lens changes shape, getting thicker for near objects and thinner for distant objects.

Presbyopia occurs when the lens of the eye is no longer able to change shape. This typically takes place around age forty. Some persons may be older, closer to fifty, and some younger, less than thirty-five, when the lens loses its flexibility. For people who have presbyopia, vision is blurred when looking at near objects, such as during reading. Also, it may become difficult adjusting focus when switching from near to distance vision.

The amount of power that is needed in glasses to correct for presbyopia is dependent on the strength of the glasses needed for distance vision. For persons who are nearsighted, removal of the glasses may make it easier to read up close. For those not nearsighted, glasses or bifocals are needed to see well up close. A complete eye examination will determine the strength of lenses needed to see well at all distances.