The Grauverlauffilter ensures that the color and texture of the sky is darkened the extent that the camera does not show the sky overexposed but with structure and natural colors. The filter with a soft gradient is up in the filter adapter and be pushed off to an ideal transition between the gray and the clear half of the filter is reached.
The Cokin P filter system fits with the matching of the diameter ago Screw on any DSLR optics and is therefore universally applicable. I recommend to pay attention to the stray light aperture is purchased add to Cokin system always to avoid the otherwise easily arise Lensflares.