1. A method of estimating an in-focus level of a target in an image scanning apparatus, wherein the image scanning apparatus comprises a first line scan detector configured to obtain one or more image scan lines of the target and a second line scan detector configured to obtain one or more focus scan lines of the target, the method comprising: obtaining at least one image scan line of the target using the first line scan detector, each at least one image scan line being obtained at a respective focus level; obtaining at least one focus scan line of the target using the second line scan detector, each at least one focus scan line being obtained at a respective focus level and wherein the focus level of the at least one image scan line is different from the focus level of the at least one focus scan line; calculating at least one focus parameter for each of the first line scan detector and the second line scan detector using the respective at least one image scan line and at least one focus scan line; and, estimating a nominal in-focus level of the target using the calculated focus parameters of the first line scan detector and the second line scan detector.