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Power of Accommodation

Put your finger a few centimeters from your eye and focus on it. You can see your finger but the distant objects become blurred. Now relax your eye and focus on a distant object, the nearby objects now become blurred. The ability to focus the eye on nearby and distant objects is called power of accommodation. 

In a camera, focusing is done by changing the distance of the lens from the film. The eye does not change focus by changing its distance from the retina. Instead, the muscles of the eye change the curvature (i.e. shape) of the lens and, therefore, its focal length. To focus distant objects, the eye muscles are relaxed causing the eye lens to become very thin. To focus on a nearby object, the eye muscles make the lens bulge out, thus reducing its focal length and increasing its converging power. There is a limit to the eye's power of accommodation. It can accommodate to see distant objects clearly, but if the object is too close to the eye, it is not distinctly visible.

For a normal eye the near point is at a distance of about 25 cm from the eye. In old age the crystalline lens of human eye becomes milky and cloudy. This condition is called cataract. This causes partial or complete loss of vision but this can be cured by cataract surgery. A normal eye has a power of accommodation which enables us to see the objects at infinity as well as at 25 cm from the eye. Power of accommodation of the eye for a person having normal eye sight is 4D.

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