Your quotation doesn't relate to my first post at all, and your advice to start by pointing out 'other considerations' was already made then!!!
In my first post I pointed out that there were no examples of softbox illumination, Point 1: The very starting point of using varying sizes of softbox demands an even coverage of each softbox to start with and the inevitable consequence of increasing size in this condition being a loss of GN.(The 'Myth'). Compared to using a strobe, which these factors are present, using a speedlight doesn't perform the same, nor result the same and only exists as its own confirmation that it doesn't change for the reasons I gave. Roberts figures bears that up well, but covering point point 1: Even coverage.. Fails completely and makes the results pointless as softbox size was irrelevant.
I even mentioned these differences in direct comparison to a strobe use, which I've tested and already illustrated.
Heres my first post:
There's not a single example of a softbox cover shown here. So the 'myth' is far from proven as the purpose of the softbox is to provide even coverage across the whole face of the softbox. The larger the softbox, the more difficult it is to obtain an even coverage.
The reason that theres no difference in the results seen between a small and large softbox here is because a speedlight facing through a softbox cover will illuminate exactly the same area when placed at the same distance from the cover in each softbox, making THAT area the effective softbox (lightsource) size.
Unlike a strobe tube which illuminates the softbox walls AND cover, a speedlight projects its light forward AT the cover and is incapable of illuminating the cover evenly - especially with larger softboxes.
Rear facing speedlights in softboxes work well (as shown by Jannie https://photography-on-the.net/forum/...3&postcount=55
) as the illumination is totally indirect. The comparable light loss is also considerably greater. In THIS example the light loss of using the Octabox is nearly 3 stops. (Based on the GN of a 580EXII being 92 @ 24mm).
Factually, using a speedlight in a softbox loses less comparable output to its unmodified state than a strobe loses. However, the coverage by the strobe will be more even and more fitting to the choice of softbox size.
Instead you seemed intent on poking holes in the original topic (intensity) with some invalid statements of your own, "Roberts post was regarding softboxes which included a single layer of diffusion ", when three of the softboxes had double diffusion.
I illustrated the same single diffusion layer, and I'm still waiting to see any samples of any other size softbox covers with even coverage using a single speedlight - especially the 6 foot X 5 foot version.
If I wanted to 'poke holes' I could find them quite easily.
As I've tried to point out - 1. Is the intention to use increasing sizes of softbox with their inherent increase in 'softness' and reduction in GN, or do you just want to 2. Blast a speedlight through 2 pieces of nylon? If your answer is 2. Then where can the Myth exist when no change exists?