In early nuclear senile cataract, the central lens nucleus becomes denser and its refractive index increases. This makes the crystalline lens optically stronger.

So, parallel rays are focused in front of the retina, producing a myopic shift, also called index myopia or “second sight.”

Therefore, we prescribe a minus (concave) lens to diverge incoming light slightly before it enters the eye, shifting the focus backward onto the retina.

Nuclear sclerosis → increased lens refractive index/power → focus anterior to retina → myopia → corrected by a minus lens.

A typical clinical clue is that an elderly presbyopic patient may temporarily read without their usual near glasses, because the acquired myopia improves near vision. This does not mean the cataract has improved: progressive cataract still causes reduced contrast, glare, color desaturation, and eventually poorer vision. [Kanski's Clinical Ophthalmology, high-myopia section] and NCBI cataract review describe this myopic refractive change.