Originally posted by Wheatfield
I'm pretty sure that this has been proven wrong on many occasions.
A very high quality filter may have a negligible effect on image quality, but a mediocre filter can have a fairly serious effect on IQ, especially sharpness and contrast, and can also affect colour accuracy and can introduce flare.
This reflects my own experience. Degradation of IQ, as far as sharpness or contrast are concerned are not detectable, if I use high quality filters. But the added flareing and ghosting is anything but rare. It is all too common, even with a good lens hood, keeping away direct light from the filter surface.
Ben