When you use the Nikon, are you having to manually dial in the distance to the subject to get the recommended settings?
The reason I ask is because I'm having difficulty understanding how the flash could reliably determine the distance on it's own.
The Canon flash is really a flash "system" and it uses data from the camera metering system and the lens' reported focus distance combined with what the flash knows. The camera can use the lens' reported focus distance to determine distance. But for an off-camera flash, it isn't safe to assume that the flash is at the same distance as the camera lens. Nor is it safe to assume the flash isn't mounted in a light modifier (when I use my 600EX-RT off-camera it's usually shooting into a soft-box ... so any assumptions about guide number would not be valid.)
I did photography for many years (decades now) using manual studio flash and even manual flash at events (weddings, etc.) with no automatic TTL systems. The math works based on the inverse-square law (light reaching the subject will appear exactly twice as bright or half-as bright (depending on closer or farther) anytime the distance changes based on the power of the square root of 2 (approximately 1.41). So if you have a subject at 10' and another at 14', the 14' subject will receive almost exactly half the light as compared to the 10' subject.
It turns out that if you divide the longer distance by the shorter distance (suppose one subject was 28' away and another was 10' away) you can get the number of stops. 28 ÷ 10 = 2.8
2.8 happens to be a full f-stop value on your camera and it is 3 stops down relative to f/1.0. That means the difference in light from a 10' subject as compared to a 28' subject is exactly 3 "stops" worth of light change.
With your actual f-stops you can set them in 1/3rd stop increments (or configure the camera to use 1/2 stop increments) but most flashes are only adjustable in full stops increments (1/1 = full power, 1/2 = one stop down, 1/4 = two stops down, 1/8 = three stops down... but there is no "1/6th power" setting (between 2 and 3 stops). So you set whichever is closest and your lighting will be within half a stop (you move the flash a bit closer or farther -- physically move the light) to obtain finer control if needed.
Changing the reflector position on the flash will of course change how much the light can scatter and that changes the guide number. This is why when you use a flash with a modifier (bouncing off a ceiling, using a soft-box, etc.) then you can't use the guide number anymore. But what you can do is test how much the modifier alters the light. You might determine, for example, that the fabric on the soft-box eats about 1 full stop of light. Once you know how much light it eats... that would be consistent for all situations when using that same modifier.
I use a Sekonic light meter that has the ability to meter flash in addition to ambient light. So I test my flashes both "with" and "without" modifiers and have my meter tell me how much I should adjust based on using that particular modifier. Knowing this, when I'm in the field I can make better decisions about flash settings and I don't necessarily have to pull out the hand-held meter because I've already tested my equipment and know the results.
Tim Campbell
5D III, 5D IV, 60Da
