In early senile cortical cataract, hydration and cortical lens-fiber changes can reduce the lens’s effective refractive power. The eye therefore develops a hypermetropic shift: parallel rays would focus behind the retina.

A plus (convex) lens adds converging power, moving the focus forward onto the retina and improving vision temporarily.

This is a refractive correction, not definitive cataract treatment. Once visual disability from opacity, glare, or reduced contrast becomes significant, cataract extraction with intraocular lens implantation is the definitive treatment. Cortical cataracts commonly cause glare as well as reduced vision, as described in the AAO cataract guidance.

Contrast: nuclear sclerosis usually increases the lens refractive index and causes a myopic shift, which is corrected with a minus lens.