Chromatic aberration

Chromatic aberration is negative phenomenon that incures midst imperfection of optical elements in objective. It especially manifests itself when photographing in very contrast scenes (tree branch on bright sky) where purple or red change-over appears on the edges of dark objects. There are methods for software elimination of this phenomenon, but the best one is to buy objective which isn't that sensitive on it.

Chromatic aberration sample photography

Graphical explanation of chromatic aberration