This paper is concerned with a novel optical diaphragm using magnetic fluid and a permanent magnet. The optical diaphragm consisting of a kerosene-based magnetic fluid and a small ring-shaped Nd
_{2}
Fe
_{14}
B permanent magnet was designed. The magnetic fluid diaphragm was driven by the external magnetic field. The experimental apparatus consists of the test magnetic fluid diaphragm system and the optical measurement system. Frequency characteristics of the magnetic fluid diaphragm in the alternating magnetic field were studied experimentally. The alternating magnetic field was generated by applying alternating current voltage to a Helmholtz coil. The alternating current signal was supplied from the frequency synthesizer. The dynamic responses of magnetic fluid diaphragm were analyzed with a high-speed video camera system. It was revealed that the amplitude of magnetic fluid hole oscillation depended on the frequency of magnetic field. The effect of the applied voltage on the amplitude of hole oscillation was also revealed.
