Why do we need reading glasses or bifocals? Distance vision is - TopicsExpress

Why do we need reading glasses or bifocals? Distance vision is good or bad, unaided, depending on whether the individual is nearsighted (myopic), farsighted (hyperopic), or neither. If the eye focuses normally, no distance vision correction is needed and the image is focused on the retina solely based on the optics of the eye (length of the eye; curvature of the cornea and the focusing power of the natural lens behind the iris and pupil), and modified by the quality of light entering the eye -- dependent on clarity of the ocular media (cornea, lens, and vitreous), and additionally affected by the size of the pupil. The act of focusing on anything at close range, on the other hand, requires an active physical change inside the eye. When focusing near (reading, sewing, using a computer, for example), the natural (or crystalline) lens must change shape to accommodate this need, by changing its surface curvature. This is done by action of the ciliary body, a ring-line muscular structure behind the iris, which by contracting or relaxing causes this change in curvature of the surface of the lens, suspended behind the pupil by very fine fibers (zonules). With progressive aging of the lens, this focusing mechanism works progressively less well and the lens becomes less elastic. For most people who see well at distance or see well at distance with glasses or contacts, a point is reached somewhere between the late 30s and mid-40s such that some external aid must compensate for what the eye can no longer do. At this point, a bifocal segment is added to the glasses prescription (or a progressive); bifocal or multifocal contacts are prescribed or a mono vision approach may be utilized; or reading or computer glasses are needed. If a cataract is removed, this removes the hardened or cloudy natural lens and there is then nothing inside the eye to cause focusing for near (regardless of age). Reading glasses may then be used or a multifocal lens implant may be be placed in the eye, at the time of surgery or mono vision may be considered. But thats another story. If you have questions , please call the clinic at 770-834-1008.

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