The focusing ability of the lens of the eye allows objects at different distances to come into clarity with little or no conscious effort on our part. However, this focusing ability definitely declines with age. The focusing power of the lens in measured in "diopters". To focus on close objects, the lens has to focus increasingly more. The diopter power of the lens is age dependent. It is not fully understood why the focusing ability of the eye declines with age.

The following table shows how the power of the lens changes with age:

A 10 yr. old has 14 diopters power
A 20 year old has 10 diopters
A 30 year old has 7 diopters
A 40 year old has 4.5 diopters
A 45 year old has 3.5 diopters
A 50 year old has 2.5 diopters
A 60 year old has 1 diopter

To read at the normal distance of 16 inches or 30 cm., the lens in the eye has to focus about 3 diopters. But, the lens is more comfortable focusing with about 1/2 of its full ability for any duration of time. Thus, from the above table, a 40 year old, with only 4.5 diopters of power available, may begin to experience difficulty reading at 16 inches or 30 cm. for any extended amount of time. A 45 to 50 year old may not be able to focus at that distance at all. It is possible to hold reading material further away than 16 inches, but eventually people will experience the phenomenon of their arms not being long enough to read! Reading glasses offer extra power so that the lens in the eye does not have to excessively focus.

If no distance glasses are needed:

* A 40 to 44 year old needs a reading glasses power of about +1.25.

* A 45 to 47 year old +1.75 reading glasses power.

* A person older than 48 to 50 may need a +2.25 or more in reading glasses.

The exact power preferred for reading glasses may vary based on what distance an individual prefers to hold the reading material.

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