Generally the magnetic field in a pickup is where the pole pieces are located either in rod or bar magnet or in a ferromagnetic material such as the steel screw pole pieces that are strongly attracted to the screws which direct the magnetic field to the strings. The magnetic field can be attracted to the mounting screws on the bottom of a pickup that’s used to secure the bobbins to the base plate. If they are ferrous, they can pull the magnetic field away from the pole pieces directly under a string. Many pickups use ferrous height adjust screws that can affect the magnetic field. Any ferrous material in or around the pickup will affect the magnetic pattern in the magnetic circuit.