The Schwarzschild metric ds² = −(1−r_s/r)c²dt² + (1−r_s/r)⁻¹dr² + r²dΩ². Photon geodesics have effective potential V_eff = (1−r_s/r)L²/r². The photon sphere sits at r=3M (in units r_s=2M=1). The critical impact parameter b_c = 3√3 M separates captured from escaping rays. The deflection angle is Δφ = 2∫dr / (r²√(1/b²−(1−r_s/r)/r²)) ≈ 4GM/bc² (weak field), diverging logarithmically near b_c.