Originally posted by jersey As guys said above - system needs to know that something changed and it uses AF points coverage change for this. With plane moving toward you it changes when it grows bigger or moves due to your movement or wind etc. Unless it happens there will be no change in AF and in your case there will be always slight backfocus.
You kind of lost me with that dissertation. What I was doing was testing the Hold AF Status technique that another member had posted for birds in flight. I wasn't advocating using the technique just reporting my findings track and shooting relatively high performance aircraft doing approaches.
Originally posted by jersey From my experience when photographing planes during landing it is best to use AF-S, not AF-C. And with center point AF or SEL-21 and focusing tied to shutter button depending on how the runway looks from your position. You press, lens acquires focus, shutter is released. It works very well even with screwdriver lenses. Unless you need FPS rate for whatever reason, then what Biz said will work.
If you are shooting with a Mk III you are under utilizing the capabilities of the system but, whatever technique works for you, I prefer tracking an inbound aircraft in AF-C and shooting short bursts then "bumping the focus" and shooting more short bursts. I usually start my tracking with the aircraft somewhere outside of 600 meters and track it all the way to touchdown. The Mk III is very capable of doing this even with screwdrive lens