Figure 4.22 depicts astigmatism, which is a lens aberration that occurs for incoming rays that are not perpendicular to the lens. Up until now, our lens drawings have been 2D; however, a third dimension is needed to understand this new aberration. The rays can be off-axis in one dimension, but aligned in another. By moving the image plane along the optical axis, it becomes impossible to bring the image into focus. Instead, horizontal and vertical focal depths appear, as shown in Figure 4.23.
Steven M LaValle 2019-03-14