How did explosive lenses make the plutonium implosion in the Fat Man bomb more symmetrical?

To achieve the extreme level of symmetry required for the plutonium implosion in the Fat Man bomb, John von Neumann utilized his expertise in hydrodynamics and shock waves to design specialized explosive "lenses".1

These lenses were constructed by combining different types of chemical explosives that burned at varying speeds—some faster and some slower. By carefully arranging these materials, the lenses were able to shape and focus the detonation wave inward toward the center of the device. This precise configuration ensured that the implosion pressure was applied uniformly across the surface of the fissile core, driving the plutonium into a critical mass without the distortions that would have otherwise caused the assembly to fail.12