The normal eye is adapted for near and far vision. This is mainly due to

The human eye's ability to focus on objects at varying distances, known as accommodation, is primarily facilitated by the ciliary muscles [c1][t3]. These muscles modify the curvature and focal length of the crystalline lens [c1]. When viewing distant objects, the ciliary muscles relax, causing the lens to become thin and increasing its focal length [c1][t6]. Conversely, for near vision, the ciliary muscles contract, which reduces tension on the zonular fibers and allows the lens to become thicker and more convex [c1][t1]. This increase in curvature decreases the focal length, enabling a sharp focus on nearby objects [c1][t4]. While the lens's convex nature is essential for refraction, the dynamic adjustment required for both near and far vision is specifically driven by the action of the ciliary muscles [c1][t2]. The iris merely regulates light entry by controlling pupil size [c2].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > 10.1.1 Power of Accommodation > p. 162
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > 10.1 THE HUMAN EYE > p. 161
3. [3] https://www.sciencedirect.com/topics/neuroscience/ciliary-muscle
4. [4] https://www.ncbi.nlm.nih.gov/books/NBK542189/
5. [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC9363677/