Comparison between the images formed according to Frensel diffraction theory and the speckle tracking approximation. In the left and middle columns, the sample has a σ width of 0.15 µm and is placed 500 µm from the focus. In the left column the sample is centred in the beam profile, whilst in the middle it has been shifted to the edge. In the right column the sample has a σ width of 0.01 µm and is placed 10 µm from the focus. First row: the exit-surface-wave intensities formed by illuminating a small Gaussian object with divergent illumination (black line). The intensities have been scaled by the factor . The sample transmission amplitudes are shown in blue. The angles along the x axis are given by arctan(x/z1) and match those of the second row. Second row: the intensity of the wavefront in the image plane (black line) and the images formed by the speckle tracking approximation (blue line). The fractional differences are shown in red. The angles are given by arctan[x/(z1+z)].