Thanks for the reply LHB!
I'm a little confused about what you said about the AF Beam and the SS and aperture? From what I understand, the AF processing (I think), goes something like this:
1. Select AF, and points.
2. Point camera and press half way down (for instance)...at this point the Aperture is wide open (in my case let's say 2.8 on the 70-100)
3. The AF Assist beam from the 600 EXII-RT is illuminated onto the subject (in the case of the 600, all of the points are supported I believe so it always turns on, or at least on most of the points). Again, Aperture still wide open
4. There are two AF sensors (or dual arrays--it's been quite awhile lol) where each one is slightly offset creating a phase difference if the image is not in focus.
5. Probably using some kind of PID loop, the AF processing stream commands the lens to adjust until the phase difference is as minimal as possible (in focus)
6. At this point the photo is considered in focus and the focus light turns on in the viewfinder and I would then press completely down on the shutter button.
It would be at this point my Aperture setting comes into play,because now the camera closes the Aperture down to the setting I have it (or the setting it chose if Shutter Pri.) The camera then fires and this is where the SS would come into play.
So I guess I don't understand how the AF Beam is related to SS and firing Aperture? Also in 1600 ISO conditions wouldn't this imply that the ambient light is somewhat darker than lower ISO's. If this is the case it would result in a more prominent AF pattern relative to the ambient light, so the higher the ISO, the better the pattern would appear for AF lock.
Maybe I have something turned on it's head, but I think that's how it would flow.
Anyway I liked your idea about DPP and comparing the focus. I haven't tried it yet but I will
I just finished a shoot a couple of weeks ago and I kept some basic "wide aperture" rules in mind and kept those fundamentals in my mind the whole time...the most dominating factor I seem to be finding is to ensure your Z-axis delta is as low as possible from the time your finger moves to the time that shutter fires. It's an easy thing to forget because sometimes the AF point is not dead on where you need it to be and you end up recomposing. At 2.8 or lower, that alone can alter the AF, but during that time not only can the sensor plane move along the Z-axis because I move slightly, but so can the subject (face, head, whatever), I found the best shots were when I did not re-compose at all and fired immediately at those apertures, If I had enough light I would go down to 4.0 or 5.6 and then had more wiggle room.
But again, there could be some additional fundamentals I am missing, or always have been that result in soft images here and there.