Comparison between effective and median Einstein radii for a cosmological population of triaxial dark matter haloes with masses M > 5 × 10¹⁴  M_⊙   h^-1 at redshifts z_l > 0.5, assuming a WMAP7 cosmology and adopting density profiles with inner slope α = 1.0. Each cross represents a pair of Einstein radii. The red dashed line indicates the bisector of the log (θ_E,med)-log (θ_E,eff) plane. The effective Einstein radii are systematically smaller, as is to be expected for ellipsoidal surface mass densities.