I have a Canon 100-400 (v1) and a 5DIII. When I attempt to shoot birds in flight, even against a blue sky with the AF set to let the camera choose the AF point, it loses focus easily.
Last time out, my settings were as follows:
Back button focus
AF case 2 (my everyday setting)
IS mode 2
focus limiter set to 6.5 M - infinity
AF points set to "all" - I figured it would increase the chances of the bird falling under at least one AF point.
I have tried selecting the AF point manually on previous outings (what I usually do when shooting other subjects) but it doesn't seem to matter and its even harder to keep the bird under the AF point. Also, I have my camera set to only show X-type AF points, which results in a narrow 3x7 vertical strip with this lens. (I learned from my old 5D that line type AF points aren't too useful)
I know the 5DIII has a very good AF system, but being full frame, the points don't cover the whole viewfinder. So what direction would you steer me in?
* Crop sensor camera like 7D/7D2 for more AF point coverage (more reach too )
* Newer lens (100-400 II, newer generation Tamron/Sigma ~50 - ~600 zoom)
* Different AF settings on 5DIII
* Practice, practice, practice