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D700 AF-ON button

Dave Spindle , May 31, 2013; 02:36 p.m.

I'm confused about the AF-ON button on the D700. If I assign AF to the AF-ON button only (not the shutter release) does the AF-ON button lock focus? In other words can I focus on my subject using the AF-ON button....let go......and then recompose and press the shutter release? Or do I need to hold down the AF-ON button while recomposing and pressing the shutter release? (I'm using Single Point AF)
Thanks,
Dave

Responses

Al Derickson , May 31, 2013; 02:55 p.m.

It locks it in the sense that focusing is only working while the AF-ON button is pressed. When you release it the lens stays at whatever focus it was. It doesn't lock it in any of the modes where focus is kept on a moving subject.

Joseph Smith , May 31, 2013; 03:58 p.m.

Dave, it works as you described it in your sentence beginning "In other words..." It works the same in S or C. I use it all the time on my D 700. In AF-S, I focus with AF-ON pushed, then release it to lock the focus, then recompose, then trip the shutter at the right moment. For something like birds in flight, set AF-C, focus on the birds as they are moving by pressing down the AF-ON button, then trip the shutter when you want to capture continuous action.
John Gerlach has a very good article on AF-ON at his web site. I suggest you look for it by Googling Gerlach back button focusing. That article convinced me to try it.
Joe Smith

Mike Blume , May 31, 2013; 05:22 p.m.

The result of using the AF-ON button in AF-S mode might be other than what you expect. Yes, a push and release of the button does lock focus to the distance of the object under the selected focus point. However if you have set the AF-S priority selection (CSM a2) to the default (Focus Priority), when you reframe the shutter will refuse to fire if the object under the selected focus point is no longer in focus. This is true for the D700 and is what makes trap focus possible with this camera. The behavior of the D800 is different.

Further, if one wishes to lock focus and then reframe (such that the object under the focus point is no longer in focus), you must press and hold the AF-ON button to allow the shutter to fire.

Hans Janssen , Jun 01, 2013; 07:21 a.m.

Why don't you try it?

Dave Spindle , Jun 01, 2013; 11:05 a.m.

Thanks, everyone. Just what I needed. Your help is much appreciated!
Hans......haven't received camera yet. Any day now! Was just reading up and getting ready.

Kevin Beretta , Jun 01, 2013; 12:23 p.m.

Focus trapping is very useful if, for instance, you want to shoot runners coming around a track at the same point every time. Or models on a catwalk. You can manually focus somewhere in mid-air close to where you are positioned. Press the shutter when the runner gets closer. As he hits focus, the camer a will fire. You need to set the camera to only shoot when in focus and AF-ON set to only focus with AF-ON and not with the half-press on the shutter. Even easier with a remote. Put the camera on a tripod and press and hold the remote when the runner is close.

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