In article , wrote:
I just tried it now with my 550EX flash, in manual mode, based on the
guide number; f/22 at 8.23 feet from flash to target with 105mm zoom.
The green channel predominates here, with blue at about 80% of the green
RAW values, and red at about 50%. The average green value in the white
square is about 1520. The blackpoint of the 10D at ISO 100 is about
126, so that leaves us 1494 RAW values for the 90% reflectance square
That would put 100% at 1660 above blackpoint. The 10D clips at ISO 100
anywhere from 3997 to 4005 or so (depending on pixel column), so the
maximum above blackpoint is about 3875. 1660/3875 = 0.43, or ISO 43, if
the standard is 100% reflectance. Now, I shot this in a white hallway
28" wide from 8.2 feet, so there is some extra light bouncing off the
walls, so the actual value is actually lower, although the practical
value in a narrow white hallway is about right.

First of all, if you want to demonstrate a 1 stop headroom, you simply
overexpose a stepwedge by one stop and compare it to a normally exposed
stepwedge.

Playing the numbers game doesn't say much.

I don't think that playing with flashes and relying on the guide number is
a good way to get repeatable results.


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