Why your cat’s eyes have slit pupils rather than round ones

Yfke van Bergen in The Times of London:

The trouble is that single-focus lenses such as those in humans suffer from chromatic aberration. This means that different wavelengths of light are focused at different distances from the lens and, as a result, some colours are blurred.

In the latest issue of the Journal of Experimental Biology , the researchers reveal that many animals solve this problem by using multifocal lenses.

These are composed of different refractive zones in concentric rings, with each zone tuned to a different wavelength.

Almost all animals with multifocal lenses have slit pupils, which help them to make the most of their unique lens, according to the paper. This is because, even when contracted, a slit pupil lets an animal use the full diameter of the lens, spanning all the concentric refractive zones, allowing for all colours to be sharply focused.

More here.