I'm trying to understand how to use the 7D's wireless flash capabilities. Specifically, when I select the [External : Built-in] wireless func
(pg 122 of the manual "Fully-automatic Shooting with One External Speedlite and Built-in Flash")
When I choose this wireless setup, there is a bracket where I can set the ratio for [external : built-in], from 8:1 to 1:1
I'm trying to understand mathematically what happens at any given flash exposure compensation setting. I still have just one control over the "overall" FEC meter. So for example, if I use the 2:1 ratio, and set the meter to the exact middle (+0) to expose for a generally well-lit subject, I'm assuming that 7D will calculate the total flash power required to achieve this exposure, and divvy up the power so that the Speedlite will provide twice the lumens that the built-in flash provides (lets assign L to the output of the built-in flash).
So basically built flash provides L, and Speedlite provides 2L
If I now change to a different wireless setup on the 7D: [One external flash only], and shoot the same subject with the same exposure (+0 FEC), can I expect the the Speedlite to output 3L lumens to achieve the same exposure as the previous scenario? Of course the quality of shadows will be different depending on the location and angle of the speedlite.
The end result I want to get from all this is to take better portraits. The 7D gives me the ability to use 2 light sources with one Speedlite, so I might as well take advantage of it. I'm thinking of using the 8:1 ratio so that the built-in flash provides a slight amount of frontal fill, while I creatively position the main light source.
Any insights on this topic would appreciated