AF-ON with AFS

Started Jan 27, 2013 | Discussions thread
blumesan
Forum MemberPosts: 67
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Re: AF-ON with AFS
In reply to rednefed, Feb 13, 2013

rednefed wrote:

I ran into the differing behavior (between D3s and D600) and lost an opportunit because of it on my last event shoot.

D3s and older cameras, AF-S + AF-ON, focus priority set:

  1. Press and hold AF-ON to focus.
  2. Camera focuses and locks on subject.
  3. Release AF-ON.
  4. Subject moves and is no longer in focus, or you recompose to make the area under the AF sensor OOF.
  5. Press shutter release all the way down.
  6. No shot. (and this is what happened to make me lose the shot)

If you want to recompose with AF-S + AF-ON + Focus priority, either the shutter button must be half-pressed before recomposing, or you need to keep pressing AF-ON. Let go of both, and you will need to focus + recompose again.

This correctly describes the behavior of the D700 and its brethren.  And since you know (or have now discovered) that the AF-ON button must be kept depressed in order to lock focus and recompose, then you will not, in future, lose any shots.

However, this behavior permits focus trapping by pre-focusing: let go of the AF-ON (or MF beforehand), recompose, and hold the shutter all the way down, waiting for your subject to cross the plane of focus.

Again correct.  And this is the function that was lost on the D800 and later cameras.

D4 and newer cameras, AF-S + AF-ON, focus priority set:

  1. Press and hold AF-ON to focus.
  2. Camera focuses and locks on subject.
  3. Release AF-ON.
  4. Subject moves and is no longer in focus, or you recompose to make the area under the AF sensor OOF.
  5. Press shutter release all the way down.
  6. Camera takes picture.

Interestingly enough, on the D4 and newer cameras, the behavior is exactly the same even when you keep the AF-ON button pressed.  So you have two ways to focus/lock focus/recompose & shoot, but no trap focus.  Not an improvement, I would say.

Therefore, the camera behaves more like Release priority. But you still can't take a picture while AF is in progress. So you are free to recompose here after letting go of AF-ON, but you lose focus trapping capability. I also haven't tested how all this interacts if you ​haven't​ initiated AF (e.g., on a cold boot of the camera) - will the D600 still click off a shot? What if you use MF override to focus?

I can see why Nikon changed the AF-S + AF-ON + Focus priority interaction (losing shots as in the first case, above) but there was no need to hardcode the change, IMO. Just one more gotcha when you switch between say, a D700 and D800.

Since you have explained how one could easily avoid losing shots with the older functionality, I fail to see any reason why Nikon made this change.  You lose trap focus and gain nothing in return.

By the way, it is not strictly correct to say that the camera behaves like Release Priority.  Once you press the AF-ON button (or even in the case where you have not initiated AF; cold boot), the focus is locked and AF ceases operation.  The lens is always focused on something although it may not be what you intended, or even anything in the frame.  The big difference (in Focus Priority) is that the D700 and earlier cameras continue to monitor the object under the active focus point (even after AF has ceased operation), and will refuse to fire the shutter if that object is not in focus.

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