Afferent objects at different distances are seen by le eye. The parameter that remains constant is

The human eye functions like a camera where the retina acts as the light-sensitive screen. For clear vision, the image of an object must always be formed precisely on the retina. According to the lens formula (1/f = 1/v - 1/u), when the object distance (u) changes, the eye must adjust to keep the image distance (v) constant. This constant image distance corresponds to the fixed axial length of the eyeball, which is approximately 2.3 cm to 2.5 cm [2]. To achieve this, the ciliary muscles modify the curvature and radii of curvature of the crystalline lens, thereby changing its focal length [1]. This physiological process, known as accommodation, ensures that objects at varying distances are focused on the same retinal plane [1]. Thus, while focal length and lens curvature vary, the image distance remains constant.

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > 10.1 THE HUMAN EYE > p. 161
2. [1] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > 10.1.1 Power of Accommodation > p. 162