CHICAGO, Dec 8 (Reuters) - More than 1 billion people globally suffer from a condition in which the eye's lens loses flexibility, blurring objects close by, and half of them do not own eyeglasses to help them see, researchers said on Monday.

In 2005, an estimated 517 million people with presbyopia -- which is thought to be caused by a stiffening of the eyes' lenses or a weakening of muscles that focus the lens -- lacked eyeglasses or had an inadequate pair.

Of the 410 million people whose presbyopia left them unable to perform tasks requiring near vision, 94 percent lived in the developing world, according to researchers from the University of New South Wales, Sydney, Australia.

Extrapolating from U.S. Census data on world populations, researcher Brien Holden and colleagues estimated the number of presbyopia cases globally will grow to 1.8 billion by 2050, adding to the demand for eyeglasses.

The condition commonly afflicts the elderly.

"Without intervention to make spectacles more accessible, the global number of individuals who will have a disability associated with uncorrected presbyopia is predicted to grow to 563 million people by 2020," Holden wrote in the Archives of Ophthalmology.

A second study in the journal predicted a jump in the number of U.S. cases of diabetes-related vision loss.

Cases of diabetic retinopathy, in which the disease damages small blood vessels in the retina, will climb to 16 million Americans by 2050 from 5.5 million in 2004, according to researchers at the U.S. Centers for Disease Control and Prevention, Atlanta.

Diabetics are also more likely to develop cataracts and glaucoma, the report said.

"Vision loss related to eye disease among people with diabetes is an important disability that threatens independence and can lead to depression, reduced mobility and reduced quality of life," wrote researcher Jinan Saaddine and colleagues.

(Reporting by Andrew Stern; editing by Michael Conlon)

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