Figure 8.

Schematic showing how depth of focus (DoF) and pupil radius in focus combine to influence the effective apertures in the presence of SA. Each subplot shows the defocus profile across a pupil for an optical system with significant SA (red line). In the left two panels, a radius of $\frac{1}{\sqrt{2}}•r_{\text{max}}$ is focused (this focus minimizes the WFE RMS), which is the dioptric mid-point half way between the marginal and paraxial focus. In the right panel, a region close to the axis (one depth of focus from the axis) is focused. The vertical height of the blue shaded rectangles represent either a large (left) or small (centre and right) depth of focus associated with low and high SFs, respectively. The blue lines dropped down from these plots enclose the pupil radii that lie within the depth of focus and thus define the effective pupil region (aperture) that contributes to the image.