I have a B&W 0.6 Graduated ND filter (2 stops) that I use for landscape in conjunction with the EF 17-40mm f/4 lens and 40D. I find it very difficult from the veiwfinder to determine the orientation of the filter without looking at the filter on the lens. I can see a difference in the image output with and without the filter (when comparing the same composition with same shutter & aperture). Is this normal or maybe just my eyesight?