
Optics 37
FIGURE 2.12: Converging lens, close source. Figure adapted from [61].
– Spherical aberration. The spherical shape of the lens causes rays that
are far from the optical axis to be focused much closer to the lens than
rays which come near the optical axis; see Figure 2.13.
FIGURE 2.13: Spherical aberration. Figure adapted from [61].
As a result, point light sources near the axis have blurred images. Build-
ing aspherical lenses, which don’t have this problem, is technically chal-
lenging. A common correction is to combine the spherical lens with an-
other, with a certain refraction index so that both lenses in conjunction
behave like an aspherical ...