A system for capturing a digital image of a slide, the system comprising: an objective lens (130, 16) having a single optical axis and an optical field of view; a motorized positioner to control the objective lens (130, 16); a stage configured to support a sample; at least one imaging line sensor positioned perpendicular to the optical axis and positioned in the middle of the optical field of view; at least one focusing line sensor positioned at an angle to the optical axis and positioned off-center of the optical field of view; at least one beam-splitter optically coupled to the objective lens (130, 16) and configured to receive one or more images of at least a portion of the sample through the objective lens (130, 16), and simultaneously provide a first portion of the one or more images to the at least one focusing line sensor and a second portion of the one or more images to the at least one imaging line sensor; and at least one processor that controls one or more of the stage and the objective lens (130, 16) such that each portion of the one or more images is received by the at least one focusing line sensor at a time prior to it being received by the at least one imaging line sensor, wherein a distance (h) between the at least one imaging line sensor and the at least one focusing line sensor is determined by h = square root(R 2 - (L/2) 2), wherein R is the radius of the optical field of view and L is the length of the focusing line sensor, and wherein time t for the at least one focusing line sensor to capture camera lines for focus height calculation and for moving the objective lens is determined by v*t = h/M focusing, wherein v is scan speed, and wherein M focusing is an optical magnification of a focusing path. an objective lens (130, 16) having a single optical axis and an optical field of view; a motorized positioner to control the objective lens (130, 16); a stage configured to support a sample; at least one imaging line sensor positioned perpendicular to the optical axis and positioned in the middle of the optical field of view; at least one focusing line sensor positioned at an angle to the optical axis and positioned off-center of the optical field of view; at least one beam-splitter optically coupled to the objective lens (130, 16) and configured to receive one or more images of at least a portion of the sample through the objective lens (130, 16), and simultaneously provide a first portion of the one or more images to the at least one focusing line sensor and a second portion of the one or more images to the at least one imaging line sensor; and at least one processor that controls one or more of the stage and the objective lens (130, 16) such that each portion of the one or more images is received by the at least one focusing line sensor at a time prior to it being received by the at least one imaging line sensor, wherein a distance (h) between the at least one imaging line sensor and the at least one focusing line sensor is determined by h = square root(R 2 - (L/2) 2), wherein R is the radius of the optical field of view and L is the length of the focusing line sensor, and wherein time t for the at least one focusing line sensor to capture camera lines for focus height calculation and for moving the objective lens is determined by v*t = h/M focusing, wherein v is scan speed, and wherein M focusing is an optical magnification of a focusing path.