Look for lenses where the zoom control doesnt change the filter orientation

Rotating a circular polarising filter changes the strength of the polarising effect, making skies deeper or lighter, and changing the amount of reflection they cancel out. If you plan on using such a filter then wherever possible buy lenses where turning the zoom control doesn t simultaneously rotate the end of the lens, and with it the filter, as this will change the effect. If you have no choice, set your zoom first and adjust the effect afterwards, being careful not to throw the lens out of focus in the process.