What is the curvature of the lens

In a human being and most other mammals, the first three mechanisms are not possible and accommodation occurs by the fourth mechanism only. The curvature of the lens (and hence it's diopteric or focussing power) is changed as a result of a series of neuro-mechanical changes in the eye. These changes are initiated by a trigger factar-the formation of a blurred image on the retina. This results in a electrical signal or impulse being generated and sent to the eye. This impulse cascades the changes of accommodation.