A digital optical microscope includes a primary image sensor that generates a primary image of a sample at a primary frame rate, an auxiliary image sensor that generates an auxiliary image of the sample at an auxiliary frame rate that is faster than the primary frame rate, and a controller that adjusts a focal distance between an objective lens and the sample along an optical axis in response to the auxiliary image, thereby autofocusing the primary image on the sample. The primary image sensor generates the primary image in response to the autofocusing.