Dear Don,
I think you are right. That is my understanding too. (Hmmm… perhaps 5-7 yrs ago?)
Depending on your application and desired lighting you may want to consider the following light modifier, particularly for portrait applications - LASTOLITE 36” OCTA BOX :
http://www.bhphotovideo.com …trod_Octa_Softbox_36.html
[It is my favorite modifier, with a very flattering soft, wrap around light for portrait (single, couple or group) and preferred over my Lastolite 30”, 24” & 16” Ezybox softboxes and Lastolite 12” x 48” Hot Rod Strip boxes].
The Lastoliote 36” Octabox with your Sigma 85f1.4 or 50f1.4 lenses would make for some stunning photographs not just pictures.
If you opt for the Springboks, you may want to carefully examine the front diffuser to ensure it hasn’t yellowed, which some material batches can with time.
Just some more things to think about, good luck.
Mary
P.S. You may want ask the manufacturer about the design - Lastolite's UK customer service have been quite receptive and knowledgable for any product inquiry I have had.