Question

for which part of the eye lens the length of focus changes

Answer 1

When the cilliary body focuses the end for near or far vision, it is known as power of accommodation; the lens focuses on the image onto the retina by the process of accommodation. The lens shape is changed by the cilliary bodies to become rounder for near vision or thinner to focus on distant objects.

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Answer 2

It is the second part of your eye, after the cornea, that helps to focus light and images on your retina. Because the lens is flexible and elastic, it can change its curved shape to focus on objects and people that are either nearby or at a distance. The lens provides 25-35% of your eye's focusing power.