Yet another AF-ON question

Started Apr 28, 2008 | Discussions thread
Jeff Diaz Contributing Member • Posts: 593
Focus will only be accurate if you know the subject was

in focus at the time you let go of the AF-ON button. All this is assuming that you have set the camera to use AF-ON only to activate AF (i.e. shutter release button set to not activate focus). I set my cameras to AF-C and Dynamic AF with release priority and AF Lock on set to off.

Under the above condition, to properly use the AF-ON technique, the first thing is to put the selected starting AF point over the selected target. If one activates AF-ON before doing this, then one runs the risk of starting focus on an object other than the subject of interest.

Once the starting AF point is on the target, hold down the AF button until the camera acquires focus (i.e. the target looks sharp in the viewscreen or you get green light ( o ). Now if you let go of the AF-ON button, AF is stopped and the focus distance remains fixed even when you recompose. This is the same as running the camera in AF-S single area AF -- even if the camera is set to AF-C 9 point or 21 point Dynamic AF, the focus is first achieved using the selected AF point. Dynamic AF does not start up until an initial focus on target has been achieved.

However, if one continues to hold down the AF-ON button, the camera continues functioning in dynamic AF 9, 21, 51, or 51 point3D dynamic area AF tracking. The minute one lets go of AF-ON, focus stops at the last point of sharpest focus as determined by the camera.

Now while continuously holding down AF-ON to activate dynamic area AF tracking, if one looks through the viewfinder with the right eye and keeps the left eye open to see the action outside of the viewfinder, one can anticipate when a obstacle will cross in front of the target. By judiciously releasing the AF-ON button while repositioning the selected AF point back over the target, one has effectively achieved an "AF lock on" of whatever duration and the minute the obstacle uncovers the target, pressing and holding AF-ON down reactivates AF and then tracking.

I do this all the time with basketball, soccor, wildlife shooting and maintain a 80%-90% in focus accuracy during a shoot. This technique is far more flexible than leaving the camera in AF-S single area AF or having AF tied to the shutter release button. One isn't giving up any focus accuracy once one learns the technique. I cannot realistically achieve a 100% focus accuracy rate because there are times when I'll roll my finger over the shutter release when anticipating the action and trying to get an insurance shot of something even if it is slightly out of focus.

carauction wrote:

D300, Using AF-ON for focus, with C-AF and 9 or 21 point Dynamic Area.

If you press the AF-ON button once and take your finger off, you have
a locked focus. This focus was acquired though, using a 9 or 21
point area, NOT a single point. Because of this, aren't you possibly
giving up some accuracy?

replies appreciated,


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