I visited some websites and they state that it is because the least distance of distinct vision is $25\,\mathrm{cm}$.
However, we know that the diameter of the eyeball is $2.3\,\mathrm{cm}$, so $v=2.3\,\mathrm{cm}$ and $u =-25\,\mathrm{cm}$ (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 $4\,\mathrm{D}$.
So how do we get $4\,\mathrm{D}$? And what is wrong with this approach?
Best Answer
The total focusing power of the eye is indeed much higher than 4 diopters. From the diameter of the eyeball, the lens formula tells you that to get an object at infinity in focus, you need about $\frac{1}{2.3 cm} \approx 43 D$.
Most of this power is static, and simply a property of the cornea and the lens when it is at rest. The accommodative power only refers to the dynamic part on top of this that we bring into effect by changing the shape of the lens, which allows us to bring objects between infinity and the near point in focus. Taking a near point of 25 cm (although in young people, it can be less than 10 cm), that gives a maximum focusing power of $\frac{1}{2.3 cm} + \frac{1}{25 cm} = 43 D + 4 D = 47 D$. The difference between the two, 4 diopters, is the maximum power of accommodation.