Has anyone had the same problems as this?

With a circular polarizer on the the 70/200 f4 lens and E40d, most pictures taken were out of focus, when I did a comparison test with and without the filter, the difference was huge, ones without were much sharper. All test photos were taken using same shutter speed!

The filter is a 7 day shop 'own' brand. Would a better make of filter make a difference?

I went back to the camera dealer and they suggested that as the zoom on the lens is internal this might make a difference, as the same filter is ok on the 17/85 standard lens which is an 'external' zoom. Or is this a problem with my particular lens, which is brand new!

any ideas will be welcome as I'm stumped

regards Balders