Astigmatism

Figure 4.22: Astigmatism is primarily caused by incoming rays being off-axis in one plane, but close to perpendicular in another. (Figure from [361].)

Figure 4.23: Due to astigmatism, it becomes impossible to bring the image perfectly into focus. At one depth, it might be focused horizontally, while at another it is focused vertically. We are forced to chose a compromise.

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.