So yesterday was the first chance I had this year to shoot some moving subjects with my Panasonic 100-300, which I acquired over winter. I went to the local auto-cross event, since I love cars, and wanted to practice with some panning of the 100-300 on my G6, paired with my Sirui monopod. I was shooting perpendicular to the cars, from about 90°, so I was shooting in shutter priority with a shutter speed of about 1/160th of a second, to capture some motion blur while panning. My problem was that because of the bright conditions and the shutter speed I was shooting at, the camera was using a very small aperture (in the range of f/16-22), which can affect sharpness of the image due to diffraction. In order to shoot at a wider aperture, I'm looking to pick up an ND filter for the lens. However I'm not certain if one ND filter will be enough (I'm currently looking at a 3 stop, 0.9 filter), and I'm curious if I'll run in to any vignetting issues if I stack two filters on top of one another? Any advice is appreciated, thank you! EDIT: Now that I'm thinking about it, I could also shoot in manual (base ISO, largest aperture setting at the given focal length, and my desired shutter speed). This would slightly over-expose the image, but I could dial in some negative exposure compensation to account for the larger aperture. Would this be a more desirable approach for retaining sharpness/detail, compared to stacking two filters in front of the lens?