Welcome to the forum!

As an adjunct to the Tangents blog, the intention with this forum is to answer any questions, and allow a diverse discussion of topics related photography. With that, see it as an open invitation to just climb in and start threads and to respond to any threads.

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


  • very interesting. Way beyond my expertise, but I am going to guess that its not a true ISO 50. I think its expanded like the higher ISO's to mimic a particular ISO and that is way it behaves that way. Of course I could be totally wrong. 

  • That makes sense, Jay.  On the Canon 6D, ISO 50 (as well as the 51K and 100K settings) are indeed expanded.
Sign In or Register to comment.