In a camera bag of gear that someone gave me is a plastic clip that I believe is used to fit square filters to a camera. With that came a clear square plastic filter in a plastic case that looks for all the world like an old-style focusing screen? The case says Cokin cromofilter SA A-060? I know virtually nothing about using these. The only filters I have ever really seen are the type that actually screw onto your lens. The only reason I finally figured out what the clip and the slide on glass were was because I saw something similar the other night in a Bryan Peterson video. That filter and the clip he used were a lot like this except his was bi-colored and he was using it for correcting contrast between sky and sea. The plastic on this one is patterned a bit and frosty, except it has a non-patterned clear dot in the middle. I'm thinking it's used for focusing maybe? It clearly doesn't change anything color-wise so I am wondering what it's used for? I took a look on Ebay but the only clear filter I can find like these is for making star effects. I'm wondering if this is maybe one of those?