The imaging quality of a simple lens decreases with the distance of incoming rays from the center of the lens. The configuration previously used to demonstrate chromatic errors is used here to show how the size of the focus depends on the diameter of the incoming beam.
Setting the beam diameter to 0.2 cm (i.e. the radius of the circular light source to 0.1 cm) a very sharp focus is achieved. As shown before, the position of the focal plane depends on wavelength due to the dispersion of the optical constants of the glass:
Increasing the beam diameter to 0.6 cm a much larger portion of the lens is used to collect light:
However, now the focus is much broader:
Blocking the rays in the center of the beam with a circular stop of 0.4 cm diameter
makes clear that the rays at the outer regions of the lens are focussed in a different plane as those that hit the lens in the center (see figure above):
