autofocus - Simply bad focus or too big aperture?

I have made a mistake with this image:

ISO 400 | 1/1700s | f/2

I used manual Aperture mode and set it to F/2 (now I believe this is too much). Kids were one meter away. It was light cloudy morning.

However, they came out blurred - and the trees behind appear in focus. I was using auto-focus mode, and I think the focus was in the middle of the photo - but between the kids :(

How come my camera auto-focused the trees behind with f/2 (i was expecting this aperture works for close objects?). Was this only a miss-focus, or my aperture was wrong as well? What should I do next time?

Answer

Your question seems to reflect the idea that a lower aperture number is for closer objects and a higher aperture number is for distant objects. This is not the case. Either aperture may be used to take photos of subjects whether they are near or far.

Besides controlling how much light is allowed through a lens, the aperture setting also determines how far things can be from the point of focus before they begin to appear blurry to our eyes when we look at a photo displayed at a particular size and distance.

The problem in your image is that the camera focused on the middle of the frame, which happened to be occupied by the trees in the background, rather than on the boys that were to the left and right of the gap in the middle. Depending on the capabilities of your particular camera there are several different ways to correct this. Basically you need to let the camera know what it is in the frame that you wish to be in focus. You can usually lock focus with the active AF sensor aligned over one of the two subjects and then recompose while keeping the focus locked or you can tell the camera to focus at a point other than the center of the frame.