An apparatus for estimating a position of one or more particles in a three-dimensional space, the apparatus comprising: an imaging system,, defining an optical axis and having a point-spread function that is asymmetric with respect to the optical axis; a detector,, in optical communication with the imaging system, configured to detect an image of a plane in the 3D space; a memory,, operably coupled to the detector, configured to store a representation of the image; and a processor,, operably coupled to the memory, configured to: (a) perform a wavelet decomposition of an image of a plane in the 3D space to form a wavelet map of the image; (b) segment the wavelet map into at least one region having intensity values above a predetermined threshold; (c) estimate the location of a centroid of the at least one region, the location of the centroid corresponding to the position of the particle in a first dimension and a second dimension in the 3D space; (d) determine a fit of the PSF to the at least one region using data around the centroid of the at least one region identified during segmentation of the wavelet map; and (e) estimate the position of the particle in a third dimension of the 3D space from the fit using the data identified during segmentation of the wavelet map. an imaging system,, defining an optical axis and having a point-spread function that is asymmetric with respect to the optical axis; a detector,, in optical communication with the imaging system, configured to detect an image of a plane in the 3D space; a memory,, operably coupled to the detector, configured to store a representation of the image; and a processor,, operably coupled to the memory, configured to: (a) perform a wavelet decomposition of an image of a plane in the 3D space to form a wavelet map of the image; (b) segment the wavelet map into at least one region having intensity values above a predetermined threshold; (c) estimate the location of a centroid of the at least one region, the location of the centroid corresponding to the position of the particle in a first dimension and a second dimension in the 3D space; (d) determine a fit of the PSF to the at least one region using data around the centroid of the at least one region identified during segmentation of the wavelet map; and (e) estimate the position of the particle in a third dimension of the 3D space from the fit using the data identified during segmentation of the wavelet map. (a) perform a wavelet decomposition of an image of a plane in the 3D space to form a wavelet map of the image; (b) segment the wavelet map into at least one region having intensity values above a predetermined threshold; (c) estimate the location of a centroid of the at least one region, the location of the centroid corresponding to the position of the particle in a first dimension and a second dimension in the 3D space; (d) determine a fit of the PSF to the at least one region using data around the centroid of the at least one region identified during segmentation of the wavelet map; and (e) estimate the position of the particle in a third dimension of the 3D space from the fit using the data identified during segmentation of the wavelet map.