An apparatus comprising: an optical actuation system configured to cause an optical source in a sample that extends generally along a sample plane (2816, 2818) to emit optical beams in a plurality of optical paths (2820, 2822); an interference system configured to receive first and second optical beams emitted from the optical source along different optical paths (2820, 2822), divide each of the first and second optical beams into two or more divided beams, and combine the divided beams to form a plurality of combined beams, wherein each of the combined beams includes an interference between two or more of the divided beams caused at least in part by a difference in a source phase between the first and second optical beams due to a difference between the optical path lengths of the first and second optical beams respectively relative to the location of the optical source, before their respective divisions; at least two detectors (2808, 2810, 2812) configured to each separately detect one or more of an intensity and a profile of each of the combined beams output from the interference system; and a processor configured to: receive the output from the at least two detectors; calculate a position of the optical source in the sample plane to sub-diffractive accuracy based on a center region of an image of each combined beam; calculate a set of possible positions of the optical source in a third orthogonal dimension that is perpendicular to the sample plane based on intensities of each combined beam detected at separate and different detectors; analyze the shape of an image of one or more of an optical beam or a combined beam based on a wavefront modification that varies with the position of the optical source in the third orthogonal dimension, wherein the wavefront modification is a modification of one or more of a curvature or a shape of a wavefront of at least one optical beam emitted from the optical source; and determine the position of the optical source in the third orthogonal dimension out of the set of possible positions based on the analyzed shape. an optical actuation system configured to cause an optical source in a sample that extends generally along a sample plane (2816, 2818) to emit optical beams in a plurality of optical paths (2820, 2822); an interference system configured to receive first and second optical beams emitted from the optical source along different optical paths (2820, 2822), divide each of the first and second optical beams into two or more divided beams, and combine the divided beams to form a plurality of combined beams, wherein each of the combined beams includes an interference between two or more of the divided beams caused at least in part by a difference in a source phase between the first and second optical beams due to a difference between the optical path lengths of the first and second optical beams respectively relative to the location of the optical source, before their respective divisions; at least two detectors (2808, 2810, 2812) configured to each separately detect one or more of an intensity and a profile of each of the combined beams output from the interference system; and a processor configured to: receive the output from the at least two detectors; calculate a position of the optical source in the sample plane to sub-diffractive accuracy based on a center region of an image of each combined beam; calculate a set of possible positions of the optical source in a third orthogonal dimension that is perpendicular to the sample plane based on intensities of each combined beam detected at separate and different detectors; analyze the shape of an image of one or more of an optical beam or a combined beam based on a wavefront modification that varies with the position of the optical source in the third orthogonal dimension, wherein the wavefront modification is a modification of one or more of a curvature or a shape of a wavefront of at least one optical beam emitted from the optical source; and determine the position of the optical source in the third orthogonal dimension out of the set of possible positions based on the analyzed shape. receive the output from the at least two detectors; calculate a position of the optical source in the sample plane to sub-diffractive accuracy based on a center region of an image of each combined beam; calculate a set of possible positions of the optical source in a third orthogonal dimension that is perpendicular to the sample plane based on intensities of each combined beam detected at separate and different detectors; analyze the shape of an image of one or more of an optical beam or a combined beam based on a wavefront modification that varies with the position of the optical source in the third orthogonal dimension, wherein the wavefront modification is a modification of one or more of a curvature or a shape of a wavefront of at least one optical beam emitted from the optical source; and determine the position of the optical source in the third orthogonal dimension out of the set of possible positions based on the analyzed shape.