Back focus or motion blur.

Started Dec 25, 2013 | Discussions thread
DesertLefty Regular Member • Posts: 353
Re: Back focus or motion blur.

Full Frame wrote:

Thanks, just thought that it being an advanced camera you could use single point in that situation. Will go back to AF-C at 4fps.

You can use single-point as the AF "area" as long as the subject doesn't move from underneath the selected focus point, but this is hard to guarantee unless the subject is large relative to the field of view. Most of the time it's better to use 9-point or 21-point mode to give the camera some wiggle room.

However, you must set the AF "mode" to AF-C in order to enable focus tracking. This is needed for a subject that is moving rapidly toward you or away from you or with a narrow depth of field, and especially for both.

Since, the D800 is a "pro" camera, it doesn't have AF-A as one of the choices for focusing mode. Hence, it can't automatically switch from single-servo to continuous-servo mode, like the "consumer" cameras can in AF-A mode. You have to choose either AF-S or AF-C (or MF).

The other thing that you must do, which can be unintuitive to begin with, is to put the selected focus point on the subject when you first engage AF tracking (either by half-pressing the shutter button or by pressing the AF-ON button). Whatever is underneath the selected focus point is what that camera is going to track.

If you start with the center AF point, then you are essentially using a modified "focus-and-recompose" technique as you pan the camera to follow the action.* If you use AF-C with 21-points, the camera will automagically switch focus points if it thinks your subject moved to another part of the frame. But the key point is that the selected AF point must be on the subject at the moment when you engage AF tracking.

My hunch is that the selected AF point was pointing at the wall when you half-pressed the shutter button and started panning. As a result, you got a well-focused picture of the wall. I've done this lots of times...

Lastly, ViewNX2 shows which focus point was used and whether or not the camera thought it was in focus (if not, the focus point won't be displayed). But, if the camera was in AF-S mode and was panning to follow the action, it doesn't show you what was under that point when the camera focused the lens.

* If you have lightning-fast reflexes (or if you can anticipate the flow of the shot), you can also use the four-way button to move the selected AF point to the subject's location in the frame. You're not required to start with the center AF point.

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