The focal length of the eye is changed by ciliary muscles so that a clear image of the object is formed on the retina.
This change in focal length results in change in lens power, thus known as accommodating power of the eye.
The power of accommodation for a normal eye is about 4 Dioptre.
A normal eye can see the distant objects as well as the nearby objects clearly due to its power of accommodation. A large focal length is required to see an object at a distance. So, the ciliary muscles relax and the eye lens becomes thin and curvature reduces. The focal length increases and the image is formed perfectly on the retina.
Similarly, in the case of nearby objects, the ciliary muscles contract. So, the eye lens becomes thick and curvature increases. This causes a reduction in the focal length for ideal image formation.
So, objects as far as infinity and as close 25 cm, can be focused on the retina.
Least Distance of Distinct Vision:
The minimum distance at which objects can be seen most distinctly without strain.
Range of normal vision:
The distance between infinity and 25 cm point is called the range of normal vision.
Persistence of vision
The phenomenon of the continuation of the impression of an image on the retina for sometime even after the light from the object is cut off is called persistence of vision.
The impression of the image remains on the retina for about
of a second.
Cinematography works on the principle of persistence of vision.