500 mA is not a low current in terms of using inexpensive, inverting charge pump devices and circuits. Methinks you need a separate, negative power source followed by an adjustable, negative voltage regulator (switching or linear), or some other means to adjust the voltage. Some DC-DC converters have an isolated output (owing to an internal transformer), so they can be driven from an existing, positive power source to provide a negative power supply.
I am assuming from the info you give that the control voltage to the camera for the focus and zoom share a common ground with the rest of the cameras circuitry. This seems to me unusual since the focus and angle are driven by geared down motors which should have separate wires for the motors, making a strictly negative voltage source unnecessary. But, it is what it is.