Originally Posted by Digitalis 
many years ago I did a test of filters,polarizers and UV filters using an old 50mm f/1.4 Pentax Takumar. And I have never seen an increase in resolution brought about by simply adding a filter, at any f stop. I have certainly seen the opposite where fine resolution was oblitherated by the use of a poor quality filter. I have seen improvements in contrast because of the filters coatings helping reduce flare, that's one of the best ways to improve an older lens,but modern lenses really don't need them. UNLESS you really need to protect your lenses against dust or water. And in those cases only use the best you can get.
I am completely with you. I did not say, that all lenses can be improved by using a front filter. I simply quoted a real life example of one particular lens. Even if the old Tamron 60B uses ED glass, it is a very old design and will profit from a UV filter, because of its longer focal length. Short fl lenses will usually exhibit less chromatism anyway, as the different foci for different wavelengthes are not that far spread out, as is the case (naturally) with longer and fast lenses. Also, the measurements (which where very thorough) showed clearly, that the advantage of the filter vanished, when stepping down the lens.
Originally Posted by estudleon I think that the filter (UV, SKYLIGHT, POL., etc) can improve the color transmission of the lens (specially in oldest cold lens like tamrons make the image more warm) but NEVER can improve the resolution.
Be sure that if you put any filter over your lens, the l/mm (or PPI) and the contrast will decrease, except the appearance of more contrast due to the transmission (don't be equal to contrast) improved.
The POL is another history, and it can be necessary in many times.
The UV filter in my example (Tamron 60B) does NOT improve colour transmission. It reduces the amount of far Blue and near UV light. The improvement in resolution is a natural and quite logical result of using the filter as you remove "colours" from the image, which cannot be brought to the same focal plane, as the image forming visible part of the spectrum. Just simply physics and no guesswork needed.
Ben