This focus sensor is readily usable in a system using pupil obscuration to measure the state of focus of a light beam. A scattering volumetric filter modifies a three-dimensional wavefront so that focus can be detected by a two-dimensional detector for conversion to a one-dimensional signal having values of plus, zero and minus. This diffusing filter comprises sub wavelength particles having a high index of refraction suspended in an adhesive to produce a solid diffuser. This focus sensor enables a well-behaved focus curve having a constant slope near best focus, while maintaining a slope that is either constant or monotonic at a minimal rate over an extended distance to support a long capture-range over which a focus-controlling servo-mechanism can acquire and maintain focus in the presence of corrupting target surface topography or optical defects.