How does the D40 decide which focus location to use? It always seems to choose the wrong focus area.
There is a pretty good, in depth article about this at kenrockwell.com.
Essentially, it tries to use the closest subject it can find without extra help from the photographer.
The camera makes a best guess as to what it thinks is the primary subject of a photo. Given that the camera has multiple (three) autofocus points, sometimes it guesses wrong. There are a couple ways to overcome the fact that autofocus can't always guess where you'd like the focus point:
- Manually choose the correct focus point and compose your scene so that your subject is at that focus point
- Use a single focus point (usually the center) all of the time, and set your focus using that point and then recompose your scene.
I believe cameras try to pick the focus sensor that has the highest contrast, plus magical proprietary special sauce.
You can either choose the sensor yourself (as in chills42's answer) or set it to the center sensor only and focus (half-press of the shutter) then recompose and shoot (full press).