There are a number of soft focus filters available for the Cokin filter contraption; as I recall, Cokin sells 2 or 3 types. The effect varies depending on f-stop, lens focal length, and on where, in the contraption, the filter is mounted. So you'd have some experimenting to do.
Bruce Dayton wrote, in part:
Any suggestions that would mimic a soft focus lens reasonably well? Diffusers are not at all the same type of animal.
