Chromatic aberration is a common optical problem in lenses where colours are not focused on the same convergence point in the focal plane. This tends to happen more frequently with cheaper and/or zoom lenses and at larger apertures. As a result, the image shows fringes of different wavelength colours normally red, green or blue around the edges where bright and dark sections meet.
Another cause is shooting against the light or a white background. In black and white photography, chromatic aberration is less prevalent due to the colour fringing being invisible, but it can also result in a significant blur in the picture. So particularly when photographing against a bright background, stop the lens down and use a prime lens.