Well, there is no shortage of them, that I can tell. There is Tiffen Black Diffusion/FX and Schneider Radiant Frost, which (according to David Mullen) soften the image with minimal halation added.     Not to mention a whole lot of Satins (Black Satin!), etc. Some good read here: https://www.thebroadcastbridge.com/content/entry/4071/its-time-sharpen-your-diffusion   And last, but not least, a handy diagram from Tiffen:   Someone already na