Radial spherical aberration comes from the lens not delivering the same refractive power across its optical zone. The base curve is the curvature that sits closest to the cornea and largely determines how well the lens conforms to the eye. When you choose a base curve that matches the corneal shape, the lens sits and centers better, keeping the optical pathway more uniform from center to edge. That alignment minimizes the variation in focus between central and peripheral rays, which is exactly what radial spherical aberration is about. In contrast, simply dialing in the correct overall power fixes the central refraction but doesn’t address how the edge of the lens bends light differently than the center. Aspheric designs can reduce spherical aberration, but their effect is more about the surface profile than how the lens interfaces with the cornea in terms of RSA. Pantoscopic tilt mainly affects alignment and other aberrations rather than reducing RSA directly, and increasing tilt can stray light paths rather than improve them. So, selecting the base curve that best matches the eye’s curvature directly reduces the radial variation in optical power across the lens, which is the main way to lessen radial spherical aberration.