This effect is specifically referred to as "purple fringing". It is quite a common fault. Unfortunately, the current consensus is that neither the lens, nor 'density' OR the quality of the photo sensor are alone responsible for this effect.
There are many websites dedicated to avoiding and minimising the effect.
http://www.pictureline.com/newslette...plefringe.html
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We do know that is most likely to happen when apertures of greater than f/4 are used, when a wide angle of 28mm (or equivalent) or wider is used and when the subject to background contrast is high. Since it is most likely to happen under those conditions, avoid that combination if possible
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Good news is that the effect can be removed quite effectively a lot of the time in post production, decent article from PC World about the technique:
http://www.pcworld.idg.com.au/index....pid;1585691688