Sid, I am just copy/pasting my post from FM forum.
My most preferred AF mode so far has been the Wide Area L mode. This has resulted in the most reliable focus acquisition in challenging situations. I also like it because the AF box size seems to be just about right and the AF sensitivity settings (tracking with lock-on) can be set to delayed (4 or 5) to remain sticky.
3D AF area is very good for perched subjects, slow moving subjects or even for most of the action scenes but struggles when there are foreground/background distractions or reflections and also erratic subjects like swallows, pigeons, diving terns etc.
I never even bothered to test the auto area AF initially as I just assumed it wouldn't be as good as the 3D but after trying it for a couple of sessions trying to track swallows and diving terns, it is an amazing mode for unpredictable action. the AF is more responsive than 3D. Page 600 of Nikon's manual says that the AF tracking with lock-on setting on the Z9 will be set to a default of 3 in case of 3D tracking vs 1-3 for Auto area tracking. This could explain why Auto area is more responsive vs 3D.
As to the wide area L mode - I noticed that it worked much better than any other AF area modes when there are distractions/ bright light sources. With 3D tracking or Auto area, the moment the camera picks the background, it is very difficult to reacquire focus and the box goes all over the place.
After reviewing many images, particularly frame filling birds i notice that the focus is sharp starting from the bird's eyes to all the way back to the tail but there were some frames where the beak is a bit soft and in situations like that i'd prefer an AF mode with close subject priority (Group area AF on Nikon DSLRS) So I found a workaround that seems to do the trick so far which is to use Recall Shooting Functions (RSF) to set subject detection to off (while the default setting for subject detection is set to Animal eye AF). I have assigned the RSF override to the lens fn button to turn off subject detection. When a bird approaches, i track the bird using wide area-L with subject detection on and the moment the bird fills the frame, i press the lens-fn button to temporarily disable subject detection. When i do this, the wide area L seems to work based on close subject priority (just like the group AF).
The other challenge i had was on how to have an override for lock on settings (1 to 5) in case i want to instantly override my default. RSF can be used to have a different lock-on values but Z9 only allows one RSF override at one point in time so my priority was to use it for subject detection off. Now, here's the interesting workaround for this lock-on setting problem.
I use Wide area AF on the shutter button. Remember this is the AF mode where i want to be able to keep the subject detection to on or off depending on my needs and hence the assignment to shutter button (Unfortunately, subject detection off on RSF override does not work on any other AF area modes assigned via custom control options). My default lock-on setting is 5 (sticky) so when i use shutter button with wide area L, i have a sticky AF setting.
I use AF-On button for Auto area mode - Now, regardless of what AF sensitivity setting i use as default (5 in my case) the camera always uses 1-3, so that's a responsive setting
Joy stick button is assigned with the 3D AF mode - Again, this uses an AF sensitivity of 3
Front fn1 button is set to Dynamic AF area - Subject detect doesn't apply for this AF area mode plus the default AF sensitivity setting of 5 would take effect here.
Front fn2 button set to single point AF - Again, no subject detect and sensitivity of 5
This way, i feel i pretty much have all possible permutations covered between fully automatic, semi-automatic and a total manual override
EDIT: As i keep shooting more, there are smaller learnings that i incorporate. I realized Dynamic AF area mode is almost useless with the Z9 so i have replaced it with wide area S (Fn1).
I use the video button to another custom setting "Access top item in MY MENU". This brings up my customized list of features that are ranked based on my priorities, as follows:
1. AF tracking with lock-on
2. Raw recording - To move between compressed vs HE formats
3. Power off delay - To quickly adjust the standby times of EVF
4. Focus points used - To toggle between full AF points vs every alternative points
5. AF Subject detection options - for various subject detection options
6. Image area - FX to DX - I use this rarely as i prefer to crop in the post
7. Continuous high speed - to change from 20 FPS to 15 FPS and so on
Some of the above functions are simply redundancies which i still prefer as i always come up with unexpected wildlife scenarios where my brain freezes and I'd want more than one way to get to my frequently used settings ;-). I also find the video button much easier to use and access compared to using the 'i' button.
