Under 50mm focal length hardly to use (An APS-C)

I have before buying clear that at a variable neutral density filter at a certain setting, the edges of the image are increasingly brighter than when the center. At low focal lengths, but one already sees these edges at a 5-10-fold extension so clear that an appropriate non-variable filter is meaningful.

I bought the filter for landscape photography, which I. 18-30mm do (to APS-C) mostly ZW at focal lengths. The filter for me was so useless that I have returned to. In my opinion, the filter is only useful in focal lengths of 50mm (to APS-C), because here sufficiently migrate due to the smaller picture angle, the bright edges until much later inside.