Three flashes are metered, Yongnuo YN-565EX, Neewer VK750 II, and also the Nikon SB-800 for reference. The idea is about comparing their actual power, and to show what their expected aperture results might be, in an umbrella and also direct flash (notice the shadows). Each flash is metered with a handheld incident meter, and this value is used for an actual picture to be able to judge and verify it.
All of the pictures should be equally exposed, because all were metered individually to make that be true. The actual camera f/stop metered and used for each is shown in a picture for comparison.
All pictures below are ISO 100, with Nikon D800, 1/125 second shutter. Flashes are always 1/1 Full manual power level, and zoomed to 24 mm. Each flash is placed on the same light stand.
Each flash is metered from above the brown bear's ear, to ensure from exactly the same location (incident reading with Sekonic L-308S meter).
White Balance is individually corrected on all, using the Porta Brace White Balance card in Photoshop CS6 Adobe Camera Raw. Correction leaves the card being a neutral color with equal RGB components (no color cast).
Metered in tenth stops, which are just that... A third stop is + 0.33 stop, and 2/3 stop is + 0.67 stop.
f/5.6 + 0.9 stop is almost f/8. f/5.6 + 0.6 stop is f/7.1 (2/3 stop past f/5.6), etc.
The necessary nearest third stop camera aperture setting might be up to 0.17 stops different than the true metered value.
FWIW, the purpose of tenth stops is for adjusting multiple lights. If we want fill ratio to be 1.2 stops less than main, we immediately know in our head that 1.2 stops less than f/5.6 + 0.9 stops is f/4 + 0.7 stops. Try computing that from f/7.1. :)
45" white reflected umbrella close, at five feet to fabric
45" white reflected umbrella at ten feet to fabric
Bare direct flash at ten feet
FWIW, another test with another meter in another situation verified these (24 mm zoom, direct bare flash at ten feet) as:
Yongnuo YN565EX f/8 + 0 EV, VK750 f5.6 + 0.2 EV, Nikon SB-800 f/8 + 0 EV
Bare flash at ten feet. It is the same as images above,
but now using standard 5500K Flash White Balance (i.e., not corrected further)
The Yongnuo and Neewer may have a slight blue or green tint, and it can be worse at low flash power, but you can see that it's a rather minor effect in this try. Still, it's easily corrected with a white card, and it's always satisfying to know we made it perfect. Any one speedlight will likely vary color more between low power (direct flash) and high power (bounce flash).
Re Exposure: Here the red in the flowers is very bright, and the red clipping needs -2/3 EV compensation to fall to 255 (the white card is well less than that level). The 18% gray card is just above midpoint, and it should be a little below. Not saying the light meter is wrong (it's much more complicated than that), but red really gets boosted in Flash or Daylight white balance (even more in Cloudy or Shade WB).