CCDs aren't monochromatic though: they detect a wide band of frequencies, they just can't tell the difference. High dispersion in the lens is a problem because

CCDs aren't monochromatic though: they detect a wide band of frequencies, they just can't tell the difference. High dispersion in the lens is a problem because a blue photon and a red photon originating from the same spot on the same object, hitting the same spot on the lens, will be deflected to different spots on the CCD. This causes fringing and fuzziness in the image.