I visited some websites and they state that it is because the least distance of distinct vision is 25cm. However we know that the diameter of the eyeball is 2.3cm so v=2.3cm and u =-25cm(least distance of distinct vision). Now if we use the lens formula to get the focal length of eye lens and then the power by using focal length and power relation we get some value other than 4D. So how do we get 4D?and what is wrong with this approach?