A normal eye can see objects clearly when placed at a minimum distance of 25cms. There is a smooth muscle called ciliary muscle which controls the view of objects at varying distances. It does this by changing the shape of lens.
When viewing a far object, the ciliary muscles are relaxed. From the below diagram, you may see that the focal length of the lens is at its greatest. The diameter of the muscles gets smaller when they contract. It causes the lens to flatten or less convex, enabling lens to focus light. When eye is focusing a nearby object, the ciliary muscles are tensed changing the shape and reducing the length of lens. It causes both front and back of the lens to become more convex and the eye to be able to focus.
When one changes one’s focus from far to near, the ciliary muscles quickly contracts causing the lens to accommodate. When one changes focus from near to far, the muscle immediately relaxes, causing the lens to revert to a flattened shape and vision becomes clear. This process is referred to as the accommodative process, or accommodation. On normal conditions, accommodation of the crystalline lens is smooth and effortless. But when this process happens continuously for several weeks or months, it causes blurry vision for few seconds which is referred to as near point stress causing headaches.
How to correct defective vision?
When the distance of the object viewed is closer than 25cms, the muscles get too strained that the eye is unable to focus properly.
What are photochromatic or photochromic glasses?