Back focus or motion blur.

Started Dec 25, 2013 | Discussions thread
clarnibass Senior Member • Posts: 2,048
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.

That might work if you are fully pressing the shutter button and you have Focus Priority set, so it would take the photo the moment it is in focus. The problem is, even in that mode, that it's likely to have a moment (even a fraction of a moment) between getting focus and taking the photo. But in AFS mode, once the camera gets focus it will not change it and follow anything. So you should really use AFC in a sitaution like this and whatever else is basically a matter of preference (single point in AFC can work too, depending on how you do it).

But another thing is that just because the focus point shows to be on the kid doesn't mean that's where the focus is, regardless of mode. You can see that if you try to focus on a completely uniform target (i.e. no contrast at all for AF) then it wouldn't AF. But, if there something contrasty sligthly to the side of the focus point, it will still AF to that cotnrast. The focus point is not limited only to its exact area.

In this case, the contrast between the kid and the wall is much higher than the contrast on the kid. It's very possible that the camera "chose" this contrast to focus on and in this case either the kid or the wall would be in focus and it "chose" the wall.

In this situation where you have a subject contrasty from the background, I look in the viewfinder if focus is on the subject.

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