3. Distance to the subject.
This is similar to the focal length but in short, the farther you are away from your subject the more the more the background and foreground will be in focus. These photos are taken what the same lens and same exact settings on a football field. The only thing that changed was the subjects distance from my camera which was stationary.
Canon 1DX about 60 yards away, 400mm @ f2.8
Canon 1DX, 400mm 2.8 @ f2.8 at 20 yards away.
4. Missed focus
This is the failure for you camera to lock focus on the subject and is not connected to the depth of field but simply the camera and lens not locking onto the subject. This could happen because of camera or operator error. Proper focusing would require a much longer discussion. This article is assuming that people are using camera equipment that is functional and using the right focal points and action tracking features within their cameras. The photos below are examples of missed focus.