I have wanted to try a circular mask for a while. I made this mask by cutting a piece of unexposed film with a hobby punch. Then I tried to find a subject slide to go with it. Turns out that a circular mask cuts out a huge amount of image. This makes it unsuitable for the vast majority of slides. I now consider it an option, just like portrait and landscape mounts, for when there are distractions on a slide pair that you need to remove.