Good optical performance, but unfortunately useless for SWW

I bought the filters for a 18-35mm FX Lens. He is not devoid of color distortion, but this is not 1000. Compared to the cost of the alternatives ColorCast is rather low.
I had him unfortunately return because it is useful in this version is not for 18-35, as it leads to strong vignetting due to the high version. The version with the flat version is unfortunately difficult to obtain ...