A method for estimating an in-focus position of a target using an image scanning apparatus comprising: a) monitoring an in-focus focal position of the target at each of, a seed location upon the target and an end location upon the target, wherein the end location is different from the seed location; b) conducting a pre-scan of the target along a pre-scan path, said pre-scan path comprising a plurality of locations on the target with respective focal positions calculated in accordance with the monitored focal positions at the seed and end locations, the pre-scan comprising monitoring a focus parameter at the plurality of locations and respective focal positions along the pre-scan path; c) conducting an imaging scan wherein the target is imaged along an image scan path comprising the said plurality of locations on the target and wherein during the imaging scan, at the locations along the image scan path, the focus parameter is monitored and compared with the respective pre-scan focus parameter for that location, and the focal position of the apparatus is modulated according to the following conditions: if the focus parameter comparison indicates that the pre-scan focal position was more in focus than the present image scan focal position, then adjust the scan focal position toward that of the pre-scan focal position; or if the focus parameter comparison indicates that the present image scan focal position is more in focus than the pre-scan focal position, then compare the focus parameters for the present image scan location and the previous image scan location, and either adjust the scan focal position in the same direction as the adjustment for the previous location if the focus parameter for the present image scan location is more in focus than for the previous image scan location, or, adjust the scan focal position in the opposite direction to the adjustment for the previous location if the focus parameter for the present image scan location is less in focus than for the previous image scan location. a) monitoring an in-focus focal position of the target at each of, a seed location upon the target and an end location upon the target, wherein the end location is different from the seed location; b) conducting a pre-scan of the target along a pre-scan path, said pre-scan path comprising a plurality of locations on the target with respective focal positions calculated in accordance with the monitored focal positions at the seed and end locations, the pre-scan comprising monitoring a focus parameter at the plurality of locations and respective focal positions along the pre-scan path; c) conducting an imaging scan wherein the target is imaged along an image scan path comprising the said plurality of locations on the target and wherein during the imaging scan, at the locations along the image scan path, the focus parameter is monitored and compared with the respective pre-scan focus parameter for that location, and the focal position of the apparatus is modulated according to the following conditions: if the focus parameter comparison indicates that the pre-scan focal position was more in focus than the present image scan focal position, then adjust the scan focal position toward that of the pre-scan focal position; or if the focus parameter comparison indicates that the present image scan focal position is more in focus than the pre-scan focal position, then compare the focus parameters for the present image scan location and the previous image scan location, and either adjust the scan focal position in the same direction as the adjustment for the previous location if the focus parameter for the present image scan location is more in focus than for the previous image scan location, or, adjust the scan focal position in the opposite direction to the adjustment for the previous location if the focus parameter for the present image scan location is less in focus than for the previous image scan location. if the focus parameter comparison indicates that the pre-scan focal position was more in focus than the present image scan focal position, then adjust the scan focal position toward that of the pre-scan focal position; or if the focus parameter comparison indicates that the present image scan focal position is more in focus than the pre-scan focal position, then compare the focus parameters for the present image scan location and the previous image scan location, and either adjust the scan focal position in the same direction as the adjustment for the previous location if the focus parameter for the present image scan location is more in focus than for the previous image scan location, or, adjust the scan focal position in the opposite direction to the adjustment for the previous location if the focus parameter for the present image scan location is less in focus than for the previous image scan location.