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| p.1 #9 · Diffusion cup color shift |
+1 @ bounce contamination ... maybe you can mask the diffuser(s) (for testing) so that you are only receiving direct light (or test in a neutral setting). Realizing that only a "portion" of the flash power is being emitted directly, the impact of indirect can be surprising. The "flash" only reading will still capture the flash (i.e. not ambient) that is reflected off nearby surfaces.
For me, when the EV value of direct sunlight is being reduced as the angle changes, the amount of influence the indirect overhead sky imposes becomes much more significant as the relationship between direct & indirect changes (same source @ sun) ... i.e. the direct is no long "overpowering" the indirect as dominantly. As such, I have to be "on guard" for the amount of blue that is mixed in with the "golden hour", particularly when there is a clear blue sky involved.
Littlebike's point that you are diffusing your flash to change the light direction ... also changes the relationship of direct vs. indirect influence. While most recongnize this regarding illumination levels/ratio/contrast ... for your color critical work, his point may be more viable than an initial credence might otherwise seem to be. It may not be the amount of color shift from the diffuser itself ... but the change in relationship between direct & indirect influences.
Conversely, you could flag/mask/block the direct light coming from your diffuser for comparison testing of the reflected flash influence from it's surrounding surfaces.
I made a quick black snoot and took readings 6" in front of it at three different power levels. The difference actually grew a bit. Btw, I bought these diffusers for all the flashes I have, but I've yet to use one off camera. They work nicely when the flash is mounted on the camera, but on a stand I prefer a true diffuser or a bounce.