A tunable Fabry-Pérot filter is described in which the cavity length is continuously monitored by a capacitance sensor and servo-tabilized. The filter can be random access tuned over > 50 nm wavelength range at 1550 nm with a response time < 1 ms. Finesse values in the range 100 to 500 have been achieved with insertion losses of between 3 and 8 dB depending on finesse. For a filter with a cavity length of 15 μm electronic noise is equivalent to a wavelength jitter of ~ 15 pm (rms) and the system temperature coefficient is equivalent to a temperature sensitivity of 0.15 to 0.35 pm C-1 in transmitted wavelength. The filter has been used as a spectral analyzer in laboratory test equipment, a demultiplexer, and in optical amplifiers for noise reduction and wavelength selection.