When you decrease the amount of magnetism or increase the magnetic gap between the magnetized poles and strings, you will decrease the output and tone of a pickup. The less magnetic field to the strings also decreases the amount of magnetic field movement or induced alternating current through the coil. The combination of string vibration and magnetic field passing through the coils in your pickup actually generates low amounts of electricity. The fewer flux lines the strings have to move through the coil will make the pickup sound softer and again less output or signal.

You may get a cleaner tone when you pluck the strings by keeping the pickup further away. You also need to get the pickup closer for more output when using lighter gauge strings or you need to move the pickup further away when using heavy gauge strings. Using heavy gauge strings and having the pickup as close as possible will give you the maximum output, sustain and distortion. Using light gauge strings and having the pickup far away will give you the least output, sustain and less distortion.