The lens is flexible and its curvature is controlled by ciliary muscles through the zonules.
By changing the curvature of the lens, one can focus the eye on objects at different distances from it.
This process is called accommodation.
At short focal distance the ciliary muscle contracts, zonule fibers loosen, and the lens thickens, resulting in a rounder shape and thus high refractive power.
Changing focus to an object at a greater distance requires the relaxation of the 
lens and thus increasing the focal distance.

The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens.
This index gradient enhances the optical power of the lens.

Aquatic animals must rely entirely on their lens for both focusing and to provide almost the entire refractive power of the eye as the water-cornea interface does not have a large enough difference in indices of refraction to provide significant refractive power.
As such, lenses in aquatic eyes tend to be much rounder and harder.
The lens is flexible and its curvature is controlled by ciliary muscles through the zonules. By changing the curvature of the lens, one can focus the eye on objects at different distances from it. This process is called accommodation. At short focal distance the ciliary muscle contracts, zonule fibers loosen, and the lens thickens, resulting in a rounder shape and thus high refractive power. Changing focus to an object at a greater distance requires the relaxation of the 
lens and thus increasing the focal distance.
The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. This index gradient enhances the optical power of the lens.
Aquatic animals must rely entirely on their lens for both focusing and to provide almost the entire refractive power of the eye as the water-cornea interface does not have a large enough difference in indices of refraction to provide significant refractive power. As such, lenses in aquatic eyes tend to be much rounder and harder.
Lens (anatomy)