Abstract: For driving exposure control vanes in photographic equipment in response to ariations in the quantity of light, a moving magnet type instrument has a magnet which is positioned proximate a fixed field coil wound on a two piece frame. The magnet is formed as a unitary component of magnetic material and plastic material interlocked against separation, and is arranged for displacement in bearings in the frame proximate which a magnet biassing material is arranged to urge the magnet into a position whereby the supported vane is maintained at a relatively constant spacing with respect to a fixed camera component.