Gravitational lensing convergence κ = Σ/Σ_crit is the projected surface mass density in units of the critical density. When κ > 1, multiple images form. The lens equation β = θ − α(θ) maps source position β to image position θ via the deflection angle α. A point mass produces the iconic Einstein ring when source, lens, and observer align perfectly, with radius θ_E = √(4GM D_ls / c² D_l D_s). Weak lensing (κ ≪ 1) distorts galaxy shapes coherently, allowing maps of the dark matter distribution across the cosmic web.