I used this effect a lot when printing negatives with an enlarger - exposing through a soft-focus filter so the blacks diffuse into the highlights, the opposite of using a SF filter on the camera lens. I can`t quite get it right in photoshop. . . .I make a duplicate layer, invert it to negative then apply diffuse glow, invert it back to positive and play with the opacity of the layer. It looks OK, but not quite there, any other ways of doing it?