What affects the focusing power of the eye?
Perhaps because the eye is compared to a camera so often, most people think the natural lens, located immediately behind the iris/pupil, is the agent of most, if not all, the focusing power of the eye.
It may be a surprise, then, to learn the CORNEA (the clear dome overlying the iris/pupil) does about 2/3 of the job of focusing! That is why LASIK works -it reshapes the cornea, not the lens.
The main job of the LENS is to change the normal eye’s focus from the default distance view to up close on demand. It does this by actually changing shape in response to a voluntary effort on our part. This happens so effortlessly and so spontaneously, we don’t even know it is happening -at least until our 40s when the aging lens hardens enough to cause PRESBYOPIA, the far-sightedness of middle age.
The size of the pupil is also a lesser factor in the eye’s focusing ability. While a smaller pupil cuts out excessive light, it also increases the depth of field, helping near vision