Fade/smooth out image effects?

Hello everyone! I am trying to apply an image effect to a camera, my screen is displayed by two cameras,one with the effect renders the first half, one without the second. Now the effect, greyscale e.g. ,applied to a single camera is cut off pretty sharply which is normal of course, but is there a way to smooth out these image effects? So only the center of one cameras viewport ist grey and it fades out to the sides ?
I could use multiple cameras making a stepped fade out but thats not what im looking for...

Thanks all!