In bright light, a dog’s pupils contract to a round shape, which is not surprisingly also the case for the ancestral grey wolf. But a fox’s pupils contract vertically, like those of a small cat. Is this an advantage? Absolutely. Canid eye lenses contain concentric zones of different focal lengths, and a vertical pupil can exploit all these zones even when at its narrowest in bright light. This improves the focusing of long and short wavelengths of light, reducing or eliminating chromatic blur/haze in bright conditions. In short, foxes are multifocal.
