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Flash with ISO 50

I recently set up my basement with seamless paper and practiced lighting white today.  Used two umbrellas in a bounce configuration each with a Canon 600EX-RT speed light.  Both lights in manual and controlled with my radio transmitter on camera.

In all cases, my shutter was at 1/160s (a tad below the Canon 6D's max sync of 1/180s).  I first put the camera in ISO 50 and was able to get pure white in the entire frame at f/2.8 and flash at 1/2 power.  And, not surprisingly, the same results at f/4 when at full power.  So far so good.

Moved up to f/5.6 and ISO 100.  But that was not enough; I was short by 2/3 stops.  I chose a couple other aperture and flash power combinations and found that when I crossed over from ISO 50 to ISO 100, I needed to adjust flash power by an extra 2/3 stop when working with ISO 100.

I then conducted an experiment to only use ISO 100 or higher and went the full aperture range of my lens (results below).  In all cases, when aperature changed by 1 stop, I had to only offset by 1 stop worth of flash power or ISO.

So there appears to be something interesting with ISO 50 involving flash; it's as if the amount of light output is more efficiently used by the sensor in this configuration?  Anyone else have similar results?

Results when using ISO 100 and above:

ISO 100, f/1.2, 1/8 power minus 2/3
ISO 100, f/1.8, 1/4 power minus 2/3
ISO 100, f/2, 1/4 power minus 1/3
ISO 100, f/2.8, 1/2 power minus 1/3
ISO 100, f/4, full power minus 1/3
ISO 200, f/5.6, full power minus 1/3
ISO 400, f/8, full power minus 1/3
ISO 800, f/11, full power minus 1/3
ISO 1600, f/16, full power minus 1/3
ISO 3200, f/16, 1/2 power minus 1/3


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