[wulf]

Light bouncing around in the camera. To some extent, you just have to learn the strengths and weaknesses of your lens. For example, I know that my 70-300mm displays purple fringing when pointing to details against a bright background (eg. branches against a mid-day sky) especially at the 300mm end. I can work round that by picking subjects with less strong light and staying away from the long end when I don't want to miss a shot.

You could also work on the picture in post processing (no cost except time) or investigate more expensive glass (no guarantee that you will never get this kind of thing and lots of money but possibly worthwhile if you must take shots your other lenses struggle with).

Wulf