Response curve of a 6-mode Zernike HMWS (b_k = 0.7 rad) to defocus error a₃Z₃ (all a_{k ≠ 3} = 0), using photometric apertures of radius r = 1.22λ/D. The diagonal line (red) denotes a perfect sensor with 1:1 correspondence, which is achieved by the HMWS for |a₃| ≲ 0.5. The response of the remaining sensor modes (Z₄–Z₈, grey) are well constrained about zero over the linear sensing regime, with residual nonlinear inter-modal crosstalk behaviour manifesting for |a₃| ≳ 1.