Visual accommodation is the phenomenon of varying the curvature of the crystalline lens to make possible the variation of its refractivity to adjust the images of objects exactly onto the retina. The visual accommodation is accomplished by the action of the ciliary muscles.
The nitid vision depends on the visual accommodation since, if the images are not projected onto the retina but in front or behind it, they will appear blurred. The closer an object is more the ciliary muscles must compress the crystalline lens (increasing its curvature); the more distant an object is more the ciliary muscles must relax.