Presbyopia is a condition occuring in mid-life, around age 40, when the lens of the eye naturally thickens, making it difficult to focus on material close-up. For that reason, presbyopia is also known as “short arm syndrome.”
Around middle age the lens of the eye becomes less flexible. It's ability to changing focusing distance (known as "accomodation") lessens and for most of us this process continues as we grow older.
Most people first notice difficulty reading very fine print such as the phone book, a medicine bottle, or the stock market page. Print seems to have less contrast and the eyes become easily fatigued when reading a book or computer screen. Early on, holding reading material further away helps, but eventually reading correction in the form of reading glasses, bifocals, or contact lenses is needed for close work.
This does not apply for people with myopia (nearsightedness). Nearsighted people can simply take their glasses off because they see best close-up.
For a very thorough, "medical dictionary" discussion about presbyopia, complete with graphics, please see The Eye Digest.
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