To efficiently and accurately perform automatic focusing.

An optical assembly is for directing a second illumination field toward an imaging surface along a second optical path that is different from and non-parallel with a first optical path. A sensor assembly is for receiving a portion of diffuse reflected illumination of the second illumination field from the imaging surface. The sensor assembly includes a sensor for producing a sensor output signal and is adjustable with respect to the optical assembly for reducing the spatial coherence of the second illumination field when it is diffusely scattered by the imaging surface. A control assembly is for adjusting a focal point of the optical imaging system responsive to the sensor output signal.