I am just back from a month of travel which took me and my trusty RX10iv to southern Portugal, the Dry Tortugas, and the Erie shore of Northern Ohio (for migrant birds). I am still experimenting with focus modes to get the most out this camera in different situations.
In southern Portugal, has in most places in Europe, it is hard to get close to birds. Many of the birds you see are either small and distant, or big and really distant (as in eagles flying high on the thermals). Also, at least where we were, by mid-morning there is significant heat shimmer over the fields and pastures and seashore, which makes any auto-focus mode problematic. In those conditions, I found, after experimenting with several different modes, that wide-frame tracking auto focus worked as well as anything. I was able, when needed, to pin the initial focus point down by touching the screen, but most of the time the camera locked on to the subject within a few seconds and held long enough for a series of shots. Keeping the camera in wide area tracking also allowed me to swing up for birds overhead without changing any settings.
In the Dry Tortugas, the birds were somewhat closer, and the light was blindingly bright. Again, I found that, in most cases, wide area tracking auto focus did as well as any other setting, and better than most, at focusing on the bird. It takes some getting used to, as the camera often takes a second to seek and find the most obvious target, but if the target is moving at all, as even in a preening bird, it will lock on. And again, you have the advantage of being able to swing to birds in flight without changing any settings.
But then I went to Ohio…the famous Magee Marsh boardwalk for migrating warblers and other singing birds. The birds are close…often less then 10 feet…very small and very active. And they are in dense cover…the trees are beginning to leave out, and there are always twigs and brush in the way. In those conditions, I had to revert to my preferred Expanded Spot Flexible Focus (without any tracking). It was the only way to get on the close, active, and too often particularly hidden, birds. On occasion I even had to switch to DMF and focus through foreground foliage and twigs.
By the way, when using DMF, I do it backwards. I use the focus ring to get close to focus and then half press the shutter release to kick in auto focus. Only on really rare occasions do I half press to enlarge the subject and focus completely manually.
So, the take away is that no one focus mode works best all the time. You have to adapt to the situation. And the Sony RX10iv has the options you need in almost any situation.
This will appear as an added chapter in my Sony RX10iv ebook soon.