1. A system for automatically focusing a microscope, comprising: an objective; a stage for positioning a specimen on a first image forming conjugate plane; a first focusing camera, configured for focusing, positioned on a first side of a second image forming conjugate plane at a first offset distance; a second focusing camera, configured for focusing, positioned on a second side of the second image forming conjugate plane at a second offset distance; wherein the first offset distance and the second offset distance are determined so that sharpness measurements for images of the specimen captured by each of the first focusing camera and the second focusing camera, at a same distance between the objective and the stage, are equal at the second image forming conjugate plane; a primary illumination source; an imaging device positioned on a third image forming conjugate plane; and a hardware processor coupled to the first focusing camera and the second focusing camera that is configured to determine that the specimen is in focus when a sharpness value of the specimen using the first focusing camera is equal to a sharpness value of the specimen using the second focusing camera.