DEAR DOCTOR K: Why does it become harder for our eyes to focus on close objects as we get older?
DEAR READER: The eye is like a camera: It has a lens that continuously focuses to sharpen the picture. As light rays enter the eye, the flexible lens alters its shape, allowing the eye to focus on objects at varying distances. The lens focuses light rays on the retina, the light-sensitive layer at the rear of the eye. As we age, the lens of the eye becomes increasingly inflexible. As a result, light rays focus behind the retina, rather than on it. This is called presbyopia. (Below, I've put an illustration of how the lens focuses light rays in a normal eye and in a presbyopic eye.)