Hi...by way of introduction I've been shooting semi-seriously for about 10 years, over which time I have acquired a fair amount of Canon gear. Mostly I shoot sports and wildlife, and have little experience with flash photography. To the extent that I get baffled sometimes by fairly basic questions.
For example, I read the article discussion FEC for Canon (in particular) in the CiC site: https://www.cambridgeincolour.com/tu...ra-flash-2.htm.
I got confused regarding the discussion of setting FEC and EC to achieve a certain Flash Ratio. I understand from the article and other sources that for Canon FEC impacts only the flash portion of the exposure, while EC impacts only the ambient portion. So far so good.
What I found puzzling was this:
"As an example of why EC is much more complicated than FEC, let's walk through what happens when you change the flash ratio from 1:1 to 2:1 in the above example. You will first want to dial in +1 FEC, since that's the easiest part. However, if only FEC is increased +1, then the amount of light from flash doubles while light from ambient remains the same — thereby increasing the overall exposure. We therefore need to dial in a negative EC to compensate for this, so that the exposure is unchanged. But how much EC? Since the original flash ratio was 1:1, the total amount of light using +1 FEC is now 150% of what it was before. We therefore need to use an EC value which reduces the total amount of light by a factor of 2/3 (150% * 2/3 = 100%). Since each negative EC halves the amount of light, we know this EC value has to be between 0 and -1, but the exact value isn't something we can readily calculate in our head. It's equal to log2(2/3), which comes out to about -0.58."
I get that in this case, if you had a 1:1 flash ratio (meaning flash and ambient contributed equally), and were to increase FEC +1, you would double the flash output, and the ratio would become 2:1, as the ambient would remain unchanged.
I also get that the resulting combined exposure would now be 150% of what it was before.
What I don't understand is how a reduction in EC of 1/2 to 2/3 can get the total exposure back to what it was, if it only impacts ambient exposure. Having doubled the flash output, that increased output by itself is now equal to the entirety of the original exposure. It seems you would then have to reduce the ambient contribution to ZERO to achieve the same total exposure. Which, in addition to being impossible, would also inflate the flash ratio to infinity.
It seems to me the answer to the question of how to increase the flash ratio in this example from 1:1 to 2:1 while keeping total exposure the same would be something like this: Say (arbitrarily) that the exposure coming from flash was 100 and ambient 100, for a 1:1 ratio. Total exposure = 200. We want to solve for y = flash exposure and x = ambient exposure such that x+y = 200 (total exposure the same) and 2x = y (flash ratio 2:1). A little algebra gets you x = 66.66, y = 133.33. Translated into stops, that's about +.4 stops of FEC, and -.6 stops of EC.
What am I missing? The blurb above seemed to be describing the way Canon's EC works, but maybe I misinterpreted, and it's describing the Nikon convention, where EC impacts overall rather than just ambient exposure.
Thanks
Tim