There's not a lot of science going on here. One thing led to another and (for my own curiosity) I ran a little "test" with the various soft filters I've accumulated over the decades.

Just click back and forth between the labels to see the effects.

There is no order to the list below. They don't go (left to right) from weak to strong. Also .... as these filters soften (blur) the images, pixel peeping is kind of silly (but if you must, I suppose you must).

The autofocus box was pointed at the doll. F-ftop = f4.0. Manual white balance.

Another little study of the Zeiss Softar on a Minolta lens showed quite an effect due to f-stop, on that particular filter. This was due to it's "microlenses" becoming obvious with large DOF. That web page is here: Zeiss/Minolta Examples.