If you are doing your own printing, I have a very simple, cheap and adjustable way to get soft focus results ( not grease, stockings filters etc ). It will preserve a sharply focused negative and is repeatable for different size prints. Get two pieces of 6x6 in x 1/4 in glass. Tape the edges so as not to get cut and place a few drops of baby oil between them. You can rotate them and/or move them up/down under the enlarging lens until you have the desired result. Make a test print as usual noting the distance from the lens. Move the pieces of glass around each other for more or less diffusion. Works great to remove blemishes. I had a copy of a print done that way enlarged to 6ft x 5ft for an exhibition a year ago ( not this example ). You still have a sharp negative.
Here's a sample
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