Question

what happen to our sight when our eyeball becomes too long?

Answer 1

When the eye ball is too long it leads  an eye defect known as myopia/short sightedness in which one can only see short distance object very clearly but cannot see long distance objects.

Answer 2

If our eyeball becomes too long,the retina will not remain at its original position but will shift back as the size of the eyeball is increased.

Thus, if a light will come from a near distance,it will converge on the retina but if the light is coming from infinity,the light rays will converge much earlier and will not meet at retina.

This condition is called Myopia or short sightedness and can be corrected be using concave lens (minus power).