As I’ve begun to shoot more and more top-notch lenses, I’ve stopped using filters except when there is sand/dust and/or salt spray or similar hazards. Especially with stellar “glass” like the Zeiss ZF Lenses and Leica APO lenses and Coastal Optics 60/4 APO, my working assumption has been that quality filters probably have no discernible effect, but then again they aren’t going to help things either.
The scene below shows increased veiling flare and degradation of image contrast with the filter. The problem with making a definitive conclusion: the filter wasn’t clean (but it wasn’t terribly dirty either). So I will have to re-shoot with a pristine filter to prove or disprove the question—but it’s obvious that a less than pristine filter does degrade image quality. The rollover below shows the results with and without the filter. Get filters at B&H Photo.