What is the cause of nearsightedness (myopia)?
Myopia, called "nearsightedness" in the U.S. and "short sight" in the U.K., is where you cannot see the distance in good focus but can see up close just fine without glasses or contact lenses.
In most cases, it is caused anatomically by an eyeball that has grown to a larger diameter than normal. The front, focusing part of the eye (cornea, iris, and natural lens) doesn't "know" this, so it focuses distant things to a more typical distance, but short of the actual distance to the retina at the back of the eye. That is why the default focal distance is at near and not off at the horizon.
We do not really understand why this happens although both genetic and environmental causes are likely. There has been an upsurge in myopia over the last 50 years. Some researchers suspect this is linked to the greater time spent by children indoors, concentrating on books and video devices as compared to pre-literate and outdoor activity intensive lifestyles.
_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic