Why does autofocus sometimes lock on the background even when the selected focus point is on my subject's face?

Question

I shot a series of photos with the same settings, using a single AF point placed on a person’s face. The camera confirmed focus each time, and the playback/focus display appears to show the focus point on the face. However, in some images the face is soft and the background branches look sharper.

I was using AF-S and mostly shooting through the viewfinder. Closer shots are sharp, but at a bit more distance several images seem to miss focus. Why can autofocus do this even when the selected point appears to be on the subject, and what can I do to improve accuracy for this kind of composition?

user15871 · Accepted Answer

Here are a few things about "AF points" that a lot of folks don't know:

Most AF "points" are larger than the squares used to represent each one in your camera's viewfinder. An AF "point" is defined by a set of two lines on the AF sensor that measure light coming from the same point through opposite sides of the lens.
Most areas of sensitivity are rectangular, either in a vertical or horizontal orientation. "Cross type" AF points are really two sets of lines at 90° to one another.
The AF system will lock onto the area of greatest contrast anywhere within the active AF "point(s)".

If the actual coverage area of your AF "point" includes the tree branches, it will focus on the greater contrast of the dark leaves in front of much brighter leaves in the background. This is because there is more contrast there than in your subject's face.

Nikon doesn't seem to publish details about exactly what areas are covered by each AF "point" the way Canon does. If they do, I can't locate such a map for the D750 so we can't show you a "map" specific to your camera's AF system. But the way the actual area of sensitivity is larger than the squares in the viewfinder (or on the LCD screen in live view) is very similar across brands. So is the way coverage areas can overlap with large numbers of AF "points".

Here's a map of the Canon 5D Mark II focus system. The focus points visible in the viewfinder are the small black rectangles. The areas of sensitivity for each 'point' are shown by the blue rectangles. Notice that only the center point is sensitive in both the vertical and horizontal directions, the rest are either vertical or horizontal only. The red points are smaller assist points for the center point when the AI Servo option (AF-C in Nikon parlance) is selected. There is nothing visible in the viewfinder to even hint at the existence of the AF assist points! These points help the system track moving objects in the frame.

5D2 focus map

Here's a map of the more complex system in the Canon 7D, along with a diagram of the focus array sensor and a chart that tells which sets of lines are responsible for which focus points. Notice that all of the focus points are cross type, and the center point includes a diagonal cross type point as well. If you look at the chart of the sensor array, you see that the lines for the diagonal cross are spread further away from each other, and thus more accurate, but only usable when a lens with an aperture of f/2.8 or wider is used. You can see in the lower diagram how much larger the colored areas of sensitivity are than the AF "points" as shown in the camera's viewfinder. Most other modern DSLRs with high numbers of AF "points" are similar

7D focus system

Why does autofocus sometimes lock on the background even when the selected focus point is on my subject's face?

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