Do Filters degrade image quality

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.

Note that by “filters’ I mean only the very best quality ones like the Schneider Optics B+W MRC line. Obviously, single-coated and/or primitive multi-coated filters are something I’m not going to entertain as an option, except in infrared, where single-coating is superior.

The scene below shows increased veiling flare and degradation of image contrast with the filter (mouse over and out). 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.

Mouse over and out to see with/without filter
(Canon EOS 1Ds Mark III + Coastal Optics 60/4 APO + B+W MRC filter)