Everything before and after the focal plane will be blurred, further objects more so. Normally, the camera can no longer focus on a distance of 10-20 cm.
Autofocus camera lenses measure the distance to the subject photographed, and sets the lens in a way that it passes through the focus plane. Auto focus function also use face detection.
Manual focus using the focus ring. This reliable method is popular with experienced photographers. Manual focus is primarily used for shooting low contrast subjects or in low light conditions.