I took some Christmas cash over to Borders and bought this book on model and glamour photography put out by Amherst Publishing.
The author goes into a lot of mathematical derivations for things photographic, but he hit one I had not seen before.
He claims the optimum distance for a softbox (front surface to model) is the same as the diagonal of the box. Thus for a 30 by 40 inch softbox, you'd place it 50 inches from the model. He doesn't go into WHY that's supposed to be optimum, or at least my initial reading didn't register a reason.
He also claims that at this distance the light on the model will "pop".
I presume the geometry of the light source is the basis for the "optimum" distance calculation. I just wonder if anybody knows for sure, and if there is indeed a distance at which something magical happens. I have used a softbox extensively, and I don't recall ever seeing such an effect, but I'm getting older and don't see as well.
And how do you determine if it is indeed "popping" since you are looking at the modeling light, not the strobe output unless you are a lot quicker than I am!
