Difference in squared visibility between two single-lens models with same parameter η₁ = 0.6. Both axes are in units. The inner dashed circle indicates the typical angular resolution (radius 0.25) and the outer circle (radius 0.5) twice the typical resolution. The reference model has a source radius ρ = 10⁻³ (with u₁ = 1.7 × 10⁻³), while the second model has a source 50 times larger, ρ = 5 × 10⁻² (with u₁ = 8.3 × 10⁻²). In this case the maximum difference in squared visibility is about 6 × 10⁻³, in practice well below the noise. It justifies the use of η₁ in the modelling instead of parameters ρ and u₁.