Review on Tiffen 77VND 77mm Variable Neutral Density Filter: Perfect for Camera Lenses, Black by Ken Wilson

I have a 58mm variable ND filter because the lens I want to screw it into is 58mm in diameter. The back, which screws into the camera lens, is 58mm in diameter. Then the front of the filter is larger. I've read that it's more about reducing the amount this lens blocks light in the corners of wide-angle lenses. For myself I can confirm that this front thread diameter is 67mm. I was able to screw on the JJC 67mm hood. I use this on a Micro Four Thirds Lumix 12-35mm lens. I can take full advantage of a 12mm lens without a lens hood. When I use the JJC hood, light blocking appears in the corners up to about 18mm. I use it on 20mm and up just to be sure. Please see the attached pictures. I am using this on an original Olympus M1 mirrorless camera. The reason for this is that I can take photos with slower shutter speeds to blur the water and clouds. Exposure setting is a dance between shutter speed, aperture, ISO and filter density. Even with a mirrorless camera's ability to preview the image through the electronic viewfinder and have a histogram in the viewfinder, I still need to capture and review the image to get the correct exposure.