Schematic for the gradient measured by a SH for a sharp discontinuity (top) and for a smoother pattern (bottom). In the top case, the amplitude of the discontinuity is not estimated correctly, and consequently the sum of SH slopes along a closed path C is not equal to zero. In the bottom case, the sum of slopes along the path C is equal to zero because the strong slope in affected subapertures compensate exactly for the rest of the path.