The Sima lens is a special-purpose soft-focus lens. It consists of a single plastic lens element mounted at one end of a tube. This tube slides back and forth inside a second tube that attaches to the camera. Sliding the one tube inside the other is how the lens is focused. The single element is why you end up with a soft image. When I originally used the lens, I noticed my images had a distinct purple flare to them, so I glued a Cokin filer mount to the front of the lens. A square UV filter in the holder reduces the purple tint and gives a more natural looking effect.
This isn't a lens you are going to use for every photo, but I find it a useful option for bridal portraits. This lens gives a romantic, ethereal quality to the portraits. Last month I was second-shooting a wedding for my friends at Platinum Fire Productions. We had some extra time prior to the start of the ceremony, so I grabbed my soft-focus lens and asked the bride to stand by one of the large windows in the bridal room (soft window light is one of my "go-to" setups for portraits). No need for any fill light, and no worries about the side lighting emphasizing imperfections. A minute later I was done, with the lovely example above being one of my favorites.