A few weeks ago I wrote about the beauty of using series filters with cameras. To refresh your memory, series filters are filters of discrete size ranges that fit inside special adapters that can be found to fit a variety of cameras and filter thread sizes. For example, Series VII filters are a good size for most 35mm cameras. Shown above is a special diopter from Tiffen (Vari-close-up lens) that is focusable and fits on the front of my Nikon Series E lens with a series VII to 52mm adapter. This particular close-up lens has a focusing helical that also provides adjustable magnification. With the 50 mm 1.8 lens wide open, I can get dreamy, soft images with glowing out-of focus highlights. If I stop down to 5.6 and smaller, I get more typical-looking macro shots that are not at all dreamy, as the lens aberrations in the Tiffin filter become a non-issue at smaller apertures. I also think it has something to do with the way the light hits the sensor on my D70.
You don't need the same exact Tiffen lens that I used. You can use any single-element diopter and get a similar effect. In fact, there are a number of series filters for diffusion -- they were big for doing portrait work, and depending on which filter one uses, differing amounts of image softenss will result. You can even combine several diffusion filters for extreme effects. Or, combine a diffusion filter with a diopter. You can do that with series filter holders, since many will accomodate different thicknesses of filters. Then, all you need is a series VII to whatever filter ring size that you need for your lens. I suggest finding a Series VII to 52mm or 55mm for starters, and using step-up or step-down rings as needed. It doesn't even have to be sitting on an SLR or DSLR -- how about trying this technique out with a Holga or Diana? Of course, the instant feedback of the DSLR is wonderful, and you can explore all sorts of filter permutations and apply what you learn to film cameras. Give it a try.
If you can't find a series VII or VI filter holder, drop me an email, as I have many boxes full, and can set you up pretty cheaply. I do have a few of the Tiffen vari-close-up lenses for sale at $35 each. When they are gone, they are gone!