Chromatic aberration is an optical error caused by the physical properties of a lens. Different wavelengths of light (colors) bend by slightly different amounts when passing through glass, so the lens cannot focus all colors at exactly the same point.

In photos, this often appears as color fringing or a faint purple/green halo, especially around high-contrast edges such as a dark subject against a bright background. It is often more noticeable near the edges of the frame, where light enters the lens at steeper angles.

It is not a separate physical “part” of the lens, and it is not just an illusion. It is a real lens aberration caused by dispersion in the glass.

Lens makers reduce it by combining multiple lens elements and using special low-dispersion glass, often labeled ED or similar. Better lens design can minimize chromatic aberration, but no lens is perfectly free of all optical compromises.