Nuclear senile cataract increases the refractive index of the lens nucleus. This raises the eye’s converging power, producing an index myopic shift.

So a patient who was previously emmetropic or hypermetropic may become relatively myopic and report “second sight,” such as being able to read without their prior reading glasses.

A minus (concave) lens is prescribed temporarily because it diverges incoming light and moves the focus back onto the retina, correcting that induced myopia. As the cataract progresses, the refraction may keep changing, and definitive treatment is cataract surgery.

Kanski’s Clinical Ophthalmology, Nuclear sclerotic cataract, p. 312.