email from Erik Allin:
"ANY and ALL UV/protection filters have some degree of negative impact on image quality. Some more than others. With some extremely high-quality multi-coated UV filters – typically very expensive – the IQ impact is so negligible as to be unnoticeable in the finished printed image to most people. Most UV filters exhibit some IQ degradation that can be seen to some degree in the image. Some UV filters can be quite bad."

I agree with this statement with one caveat: for a high quality filter the IQ degradation is usually not measurable within the noise of the image except in rare circumstances. I'll remove protective filters when I'm doing photography that includes bright subjects, like a moon in deep twilight, or lightning. I do a lot of night city scene images, especially when I'm on travel. (e.g., Hey Jay, check out my Sydney Opera House image at http://www.clarkvision.com/photoinfo...ht.photography it was used by a Sydney advertising firm--funny, it seems like they could have gotten their own image). Since switching to multi-coated filters, I've not had a problem, so sometimes I forget to take off the filter and have never had reflections or other problems.

But this thread has been illuminating. I'll try some tests over the next few weeks with filters and see if I can find problem filters. I have a variety of test methods that I can design that anyone can use.

The problem people seem to be observing with filters on telephoto lenses is probably due to the following. Filters are very thin, and on small lenses, the apertures are very small. A filter must be flat and uniform to at least 1/2 wavelength of light--that's about 1/5 of a micron. A 50 mm f/1.4 lens has an aperture of 35 mm but image quality is limited by other aberrations. Stop down to f/8 and the aperture is only 6.25 mm, so the filter performance needs to be 1/2 wave only over a 6.25 mm spot, which is fairly easy (good quality window plate glass meets that spec). But a 400 mm f/5.6 lens has a 71 mm aperture, so the 1/2 wavelength criterion is over a much larger area and harder to achieve. It is this larger area of required flatness that probably creates the problem, rather than the magnification of the long focal length.

So again, if anyone has a filter that they believe is causing a problem of soft focus, I would like to examine it. I'll even return it if you wish. Contact me by email or PM.
Roger