Autofocus can feel very natural when it does its job and very confusing when it doesn’t. You press the shutter button, the photo seems fine for a moment, and then a bit later you realize that the background looks sharper than the person, item, or spot of detail you meant to photograph. The camera has focused, on something.
This kind of thing commonly occurs with multiple visual objects or activities taking place in the frame at once. The phone camera may lock onto a brightly colored object in the foreground, a distinct, high-contrast line, or a face in the background; the camera camera may focus on a stick or branch, the edge of a table, or a sign or other text in the background, or some pattern with a higher contrast level than your real subject. The camera is finding what it can easily read.
One useful test you can do is to set up a single, plain subject on a table or desktop and add a few items behind the main subject. Try to use window light, and make sure your camera is on a fixed tripod or some other stable support. Shoot one image without using focus tap on your phone or focus point selection on your camera. Next, tap your main subject on your phone’s viewfinder screen or move the focus point to that point using the camera, shoot again. Now compare the images by cropping to the main subject, then to the background in each photo. You may notice little visual difference, but it is these little differences that you will start to notice.
Look for visual cues that your focus is not where you expect it to be. The subject of the photo looks a bit soft, while a book spine, a fold in a curtain, a leaf on a plant, or even something in the foreground such as a mug is sharper. The eyes of a person look less sharp than hair, clothing texture, or the wall nearby. The front edge of a still life subject appears a little softer than an item behind it. These details are important, rather than how the photo looks overall.
It is more of a problem with closer subjects, because the distance to the background is usually only a little bit more than the distance to the subject. Focus distance is an issue that is noticeable with just a slight error. Try to move back a bit from your subject and select your focus point again. You may also try to clean up or simplify the background, so there are fewer objects that attract the camera’s attention.
Light and contrast are also involved. When a dark subject lies close to a bright window, it may be difficult to read and easy for the camera to focus on the window frame behind it. If your subject is a uniform surface with little detail, you may need a more distinct focus target, such as an edge, an eye, or a label. Before you press the shutter, you should know what part of your subject should appear sharpest. “The person” is a little general; “the nearest eye” is more specific. “The object” is too broad; “the edge nearest the camera” defines the area in a better way.
End the photo with three images; do not scroll through all of them. Select the one image where you can see that the focus fell in the intended place, one where you can clearly see that it did not, and one where you are less sure of where the focus fell. Say aloud what the camera focused on in each image, and note the pattern. In time, these kinds of focused photo review exercises will help you understand what the camera is choosing to focus on so that you can start to spot and avoid the bright objects, detailed backgrounds, and close distances that will trip autofocus up.