When you add another piece of glass not intended by the manufacturer (especially if it doesn't fit exactly - and you seem to have one smaller than the filter ring which is also giving you vignetting) you will usually have something called chromatic abberrations. This also happens with cheaper really long or really short focal length lenses. This is when the glass is not refracting the individual colors to the same spot - the Cyan, Magenta, and Yellow are misaligned on their projection points (digital sensor or film). Ever wonder why higher-priced really long (and really wide) lenses have APO in their names? The glass has been designed and calibrated to not give you the chromatic edging you see in your above images. Sorry to say, you're going to have to live with the edging - the flip-side is that now that you know what it does, pull it out and use it to effect the mood of the shot you're taking. It can convey something "dreamy" or the like. Play with it - have fun. I know a guy who modified a piece of a Schneider glass element to one of his lenses to get the middle in focus and almost a radial blur with REALLY BAD chromatic abberations spreading out - he used it for "dreamy" wedding shots - not my cup of tea but his clients seemed to love it! Hope that helps- -Lew ;-)