With reference to the discussion on DPChallenge, with regards to filters on the 100-400mm IS L lenses.
Both were taken on a Canon 100-400 IS L Lens at 400mm, however, the left photo had a standard Hoya (Green Box) filter, and the right had no filter. I did a lot of testing, as I thought I had bought a fawlty lens.

You can see the soft focusing and double outlining appearing around the leaves. In some photos is looked like the background was blur, like an action shot, but the subject was stationary.

I am not saying the filters are bad, they just didnt go with this L lens, as I use the filters on my other cheaper! lenses.

Both photos are on highest Canon 300D quality output, but scaled 50% and sharpened.