When the object is at infinity, the parallel rays from the object falling on the eye lens are refracted and they form a point sized image on retina
In this situation, eye-lens has a maximum focal length.
When the object is at infinity, u=-∞; v=2.5 cm (image distance which is equal to distance between eye-lens and retina) using the formula
we get, consider that an object is placed lat distance of 25 cm from our eye. In this situation eye has minimum focal length.
If the position of an object is between infinity and the point of least distance of distinct vision, then the eye lens adjusts it's focal length in between 2.5 cm to 2.27 cm to form a clear image on the retina.
The ability of eye-lens tochange its focal length is called"accommodation of lens".
Sometimes the eye may gradually lose its ability for accommodation. In such conditions the person cannot see an object clearly and comfortably. The vision becomes blurred due to accommodation defects of the eye lens. There are mainly three common defects of vision.
They are:
i. Myopia