Just noticed this thread about diffusers for macro. It is good to keep discussing this topic. John is right about the difficulty of quantifying diffused light.
Also, in my search for the ideal diffuser for macro, I have found that one diffuser does not fit all. Ideally, you would want a different diffuser for a shiny bug (for instance a beetle) compared to a bug with a dull appearance (moth). The degree of magnification also affects how light from a particular diffuser looks.
But when we are in the field, there is usually little to no opportunity to tailor the lighting rig for the subject. At least that is my experience. I load the rig up with my flash and modifier and shoot what I find. That means (for me) that there will be variance in the results.
But I accept the compromise. Where I live, we have bugs for only about 4 months of the year, mid-May to mid-September. Therefore much of my bug photography takes place on trips to warmer places. And that imposes another compromise: the gear has to be portable for travel. My solution, as mentioned in other threads, has been to use a fan-shaped diffuser on the end of my 100mm L with a snoot on a camera-mounted flash. The materials collapse flat and weigh practically nothing. They give satisfactory results at times IMO.
Here are a couple of examples from my last trip, which was to Cuba.
Lygaeus sp bug, looks OK to me.
Iridescent fly, same flash rig, but too hot I think. (And a bit OOF
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